BMW Airhead Motorcycle Brakes
(including conversions)
©  Copyright, 2011, R. Fleischer
brakes.htm-53

***Broken cable or lever operated brake switch (NOT the hydraulic switch)......see article on brake switches-37B

 NOTE!.............regarding the hydraulic pressure activated brake switches.   Those switches, originally 34-31-1-233-959, were replaced with 61-31-1-244-334,
 and you can probably substitute the switch used on old VW Beetles.  Napa carries that switch as number SL143.

WARNING #1!! 

When cleaning a brake caliper during an overhaul, it is somewhat common practice to separate the halves (or not), and to clean the calipers, using Brake Spray Cleaner, and if lots of deposits and mechanical means do not clean them out....to use a spot flame and burn the deposits, and THEN clean them out.   This is a VERY important caution....never, EVER, EVER, heat Brake Cleaner itself.   The reason is that high heat, on the liquid brake cleaner, may create the intensely serious poisonous substance called PHOSGENE.   Phosgene is produced when chlorinated hydrocarbons are excessively heated.  I am not in favor of splitting the caliper halves, most of the time.

When I have a 'problem' caliper to clean, and regular solvents don't work, then I clean them the best I can in water with detergent added.  I then use a propane torch on the deposits....then mechanically clean the caliper with stiff brass bristle brushes, ETC....I try to get every remnant of mineral and other deposits out of the caliper...especially the sealing square O-ring area. DO NOT use a propane torch on wet "BRAKE CLEANER", or, any other hydrocarbon solvent.

NEVER, EVER put pressure on brake discs when changing tires or doing other wheel work!  Support the wheel with, perhaps, pieces of 2 x 4 lumber under the tire/rim.  Keep the disc(s) OFF the ground!  If you do not heed this warning, you MAY warp the disc/carrier.  Some folks have old oil drums, one end cut out, set vertically, for use as a fixture when working on disc wheels.  

Warning #2:

Brake bleeding should be considered a ""NORMAL YEARLY MUST DO"" maintenance item. It is usually easy to do, and hardly has any problems,  if you do it right!    Failure to annually (or at least every other year at worst case) perform a full system bleeding, in which all of the old discolored fluid is properly bled out of the system and replaced by fresh fluid, will eventually cost you a lot of money, and can cause an accident if bad enough.   Once things start to get bad due to neglect, they tend to get worse at an increasing rate.    This will cost you $$$ eventually.  Failure to change the fluid and bleed the brakes may well eventually cause the brakes to bind, seize up, and MIGHT toss you over the handlebars at a very inopportune time (I think ANY time is bad for that!!).  See #9 below.
 

Frozen or jammed pistons; retraction, etc:

In some instances, you might find your brake caliper pistons jammed; or, partially so.  More subtle (and seen more often) is to have them not retract as well as they should, and that can let the pads stay in contact with the discs with too much pressure when they should hardly or barely be touching.   There are usually two causes for these things.  One is the O-ring (yes, it is square in cross-section) that is around the piston and seals the fluid from escaping.  That O-ring, by deformation (it is designed to do that), is what is supposed to help retract the pistons ever so slightly, once you release the brake lever pressure.  These O-rings have been known to get hard with age....but it is the corrosion in their associated caliper groove that is typically the main problem.  Water eventually accumulates from absorption, to the extent that the fluid contains a fair percentage of moisture, and with the corrosion, the O-ring and piston will have sticky problems.   Another problem with old fluid...or just accumulated carbon and rubber residue... is that they can plug up the small return hole in the master cylinder, thus the caliper part of the system will retain pressure, and the pistons will not retract fully.    See #9 below.
 Still another problem is a faulty hose, from kinking the tiny diameter internal tube inside those hoses.....that can create a one-way valve.

Longevity of BMW Motorcycle Brake Hoses:

Although this particular problem used to be very rare, it is not all that rare nowadays.   The BMW stock hoses are VERY good and LAST for quite a few DECADES; ....IF....... you do not hang them by the calipers.  That BAD practice CAN excessively bend the hoses INTERNALLY at the fittings ends...this is almost always damage that is internal and hidden.   The small diameter internal stiff plastic tube (the thick rubber hose covers that internal tube) kinks, and produces a partial one-way flap. 

There is a LOT of pressure from the master cylinder when using hand or foot pressure.  That pressure needs to relieve itself when the lever is released...by a relatively small amount of fluid coming back to the master cylinder.  AFTER the major part of that pressure is relieved, one hopes, by releasing the lever, the final bit of pressure relief effect is from the caliper piston's O-ring deformation, now pushing the piston back a really tiny amount.   If a flap or kink develops in the inner tube of the hose, the pressure maybe only partially relieved, at best.  Even a partial non-release will cause problems, the least of which is, after awhile, a change in surface of the pads, and thereby some squealing and possibly poor pad friction. The pressure might be released very slowly, or not at all, or incompletely.    The worst situation would probably be enough heat from enough pad friction to allow the possibility of brake fluid boiling (especially if the fluid is old!), causing bubbles, leading to NO BRAKES after you are riding awhile !!!  I suppose one could argue that sudden freezing-up of the brakes in a full-on situation would also be a worst situation.

Stainless Steel braided lines are NOT generally any better overall and potentially FAR WORSE, and I recommend you do not willy-nilly replace stock hoses with them.  There are exceptions, but that is a good general rule.   I discuss SS hoses in more depth later in this article, at #10.   Many aftermarket Stainless Steel braided hoses have lousy internal construction, and some are so bad that the fluid return is not fast enough.

Brake fluids for use in BMW Motorcycles: 
(see also item 21 near the end of this article!)

In MY opinion, the absolute limit for brake fluid changing for a bike with very MINimal use, and stored in a closed garage, is 24 months. 
 Those who ride in a lot of humid weather, or who ride a lot; if the bike is parked in the sunlight, etc.,  might even consider 6 month fluid changes.  Fluid is VERY cheap compared to new parts!

Use ONLY DOT3 or the slightly better temperature range and moisture capability of DOT4.  Keep it OFF the paintwork, and keep a VERY WET RAG on your nearby paintwork when working with it.  If, HORRORS!.. it gets on the paintwork, wash it off with water,  INSTANTLY...that means RIGHT NOW! You do NOT have 10 seconds!   ALWAYS have a WET RAG (and maybe a container of water) instantly available when using brake fluid!  

Don't use "race" fluids...they require changing once or twice EVERY year, and offer NO advantages to a street bike.     There are so-called 'premium' DOT4 fluids; some are called RACE fluids; and there is also, confusingly, a DOT 5.1 that is NOT silicone based and hence is usable...but WHY?...I HIGHLY recommend against anything but DOT3 and DOT4 ...yes, I recommend the standard common inexpensive fluids.

 I approve of inexpensive DOT3 for most folks and who do change fluids EVERY year (two years the LIMIT!) and who are not overly hard on brakes; and definitely recommend DOT4 for those who are hard on the brakes, getting them VERY hot.  It IS BEST to change brake fluid yearly.    I personally do it as part of my pre-Winter service.    For longest brake system life, your brake fluid really needs yearly changing, as it attracts moisture, right through the non-leaking lines, molecular sized holes in everything.... but especially the master cylinder bolt and screw fastenings, etc.    Bleed the brakes until clear fluid comes out. Best to use a fresh 8 ounce can each time; but that recommendation is not mandatory, as much as many seem to think...just don't leave the cap off for appreciable amount of time.  ...and that includes the master cylinder inner boot and cover.  Open the container only when you need to actually pour some brake fluid into the MC.   If your MC has an internal bellows-boot associated with the cover, clean it with a clean lint free rag...don't wash it in water unless it is a mess...and if so, DRY IT.   DO leave it OUT on a CLEAN rag or paper, until you are all done.  While doing the bleeding process keep the MC COVER in place UNLESS FILLING the MC.   That is all the protection against humid air you need during bleeding.   You will probably have brake fluid left over.  It is OK to use it the following year, provided it WAS CAPPED in the meantime, AND, it is still CLEAR fluid.   I have thrifty friends who use old brake fluid for stripping paint!     

If you regularly bleed/change fluid until fluid is nice and clear, and don't hang the calipers by the hoses, then you are UNlikely to EVER have to replace the master cylinder or calipers or brake hoses.....these can last for decades!  

NEVER use DOT 5 silicone fluid in your BMW braking system...NEVER!  DOT5 silicone fluid does not absorb moisture, so it allows moisture to condense into droplets in your braking system and thereby probably rotting it out faster; and it can, in freezing weather, FREEZE the brakes!  In most situations yearly bleeding will NOT get trapped moisture out of a silicon fluid system.    Silicon fluids are also generally not compatible with most of the rubber parts in your bike's braking system (depending on year of seal manufacture, and if Brembo or ATE).    DOT5 silicone fluid has no advantage for Airheads.  Silicone fluid is thin, and does not absorb water vapor like DOT3 and DOT4 and DOT 5.1 does....so you MIGHT think that it is better and does not need bleeding, and if it did, would be easier to bleed on an opened system...but, the truth is that silicone brake fluids tend to get millions of tiny bubbles, defeating that idea; and, while it won't absorb water, water does gets inside from various means, and forms globules, and corrodes the parts!   At very elevated temperatures, silicone gets compressible!.....a BAD thing.      Yes, there ARE reports of folks using silicone brake fluid for years in Airheads, with no problems.  I suspect that there are various types of rubber parts in the system in use over the years.  Brembo themselves has always used 'natural rubber' (per Brembo) that is not compatible with silicon fluid.  That is how it probably STILL is.  I am unsure on ATE calipers.   Best not to be a Test Pilot.  

Remember:  moisture droplets can do REAL FAST damage, but small to moderate amounts of moisture dissolved in DOT3 or DOT4 acts so slowly that yearly fluid changes really do work well! 

Calipers and master cylinders on BMW Airhead motorcycles, working on the system, etc:

If you DO open a system, for such as rebuilding a caliper, or changing a hose, you likely will see all sorts of recommendations that you use only brake fluid or specific brake cleaner, for cleaning parts.   You COULD use other solvents, heck, even detergent and water...but after drying, any reassembly means using brake fluid for lubrication of parts during assembly, ETC.  

There is a product called by various names, but it is a special grease for use in assembling the calipers.  Some brake kits come with a tiny tube of the stuff.  USE IT, otherwise you may have a lot of fun trying to get the pistons into the calipers.

The rear master cylinder repair kit, up to 9/1980, is  34-31-1-237-233; thereafter is 34-21-1-242-791.

The 40 mm ATE swinging caliper repair kit is 34-11-2-301-709 (38 mm is -705).  I don't think that kit has the INternal O-ring, that is needed if you separate the halves.  Reportedly, the EPDM R-ring part 9557K72 from McMaster-Carr fits and works OK.
The early ATE calipers were quite different, and incorporated a strange looking flat spring and an O-ring, at the pads.  These were anti-rattle, and anti-squeal parts.   O-ring 34-11-1-233-120; spring 34-11-1-232-513.   

The Brembo 38 mm caliper repair kit does not include the piston.  The kit has just the O-rings and cap seal.     That kit is 34-21-1-237-234.

RE:  FOR THE OTHER ATE PARTS:   BMW sells only the full kits.  For individual parts, you can ask around; try Motobins of England, and try www.capitalcycle.com in the USA.

If overhauling calipers, it is best that you do NOT take the halves apart, if you do not have to.  If you DO take them apart, you must replace the O-rings, and you must have the halves perfectly clean, smooth, and dead flat.   BMW does not supply the halves-sealing O-rings, which you will then have to obtain from some other source.   If you are re-bolting the halves, they must be tightened evenly, and a torque wrench used. 

 Caliper pistons work best if fairly well-polished. 

BIG HINT!:  If you are overhauling caliper internals, you may find that cleaning/scraping old corroded caliper bore grooves can be done by some sort of prepared screwdriver blade, or one can do it a bit easier by heating the bore with a small gas torch....to hot, not fiery!...once cooled, the residue will be much easier to clean out.   If your grooves clean easily, forget that.  NEVER EVER have wet brake cleaner in the caliper, when heating with a torch.  I don't use brake cleaner for cleaning calipers.

There are places that can resleeve/overhaul master cylinders.
Try:  www.applehydraulics.com
        www.brakecylinder.com
        http://www.autosportseattle.com/
        S A Master Slave, Wynberg, Capetown, So. Africa    2721 761 6962
       

Check my REFERENCES page under brakes for more places.  Yes, that was a link to that page.

All the rest of the details:

1.  DRUM rear brakes; shaft O-rings...etc.:   There is, generally after 1980, up to 1985 (??), one or more rubber O-rings on the brake actuating shaft, in the SQUARE grooves in that shaft.  That shaft may have up to 7 grooves machined into it. Do NOT put any O-rings into the round cut grooves.  Prior to 1980, most did not have any O-rings, you simply cleaned the shaft, greased it, and there was a felt just inside the right side actuating lever, at the spline end.   I like to use silicone grease here,  on the shaft (and O-rings if has them). 1981-1984 (to 3/1983??) models used 4 O-rings, in the square cut grooves.  The early tube that the brake cam passes through was different than the later ones.  The early one fit into the cover, and leaks were possible.  In 1985 (may be an error, could be 3/1983)  BMW changed to a full-length tube through the rear drive, and the shaft is inside that tube, and all you have to do is to be sure to grease the shaft. You can use conventional petroleum grease for that.    Some anal types, and I am one of them, feel that the shaft of any of these models should be removed, O-ring(s)  freshened if shaft uses them, and clean and freshly greased, at EVERY tire change... or every other tire change.    It is possible to fit that tube to the 1981-1984 (to 3/1983?) models, but it isn't really needed.   The brake cam lever that has those grooves can be updated, but you will likely have major problems getting the correct one, so that is why I say to just change the O-rings.   NOTE that if the rear drive housing cover is disturbed, that tube must be realigned, or you will have recurring oil leaks. Do NOT allow the cover to be misaligned.   Try an old /5 front axle for this alignment.   
 Sometime, perhaps after 1983 (3/83?), BMW modified the brake cam rods.  The O-ring grooves became shallower, and were spaced differently.  It is all rather confusing, and trying to upgrade/update can be a mess.  I suggest that all of the cam rods work fine, if you replace the O-rings...and lubricate the shaft... at tire changes. 

1981+ rear drum brake pivot shafts, the shaft that has the O-ring(s), will not accept the O-rings, and it will be a bear to install them, if the diameter measured at the bottom of the grooves is not a minimum of 10.41 mm.

There was a rear brake cam change as of 08/1989 production.  The early O-rings were 10 x 2 mm, and were  36-21-1-239-134.  Later, BMW changed that ordering number to 07-11-9-906-328.  AFTER 08/89, the O-ring became 35-21-1-457-605, which were 12 x 1 mm.  The early O-rings of 10 x 2 will NOT fit the later cam.

****Drum brake linings:    www.vintagebrake.com   Michael (Mercury) Morse....is a great resource...and can even reline your brake shoes with modern material....and arc the shoes to fit your brake drum, ETC.!    Ask Michael about sending your drums (ask him if he can handle the wheels?...which would be better, drums distort faintly when re-spoked)...or you can precisely measure the drums and give him the measurements(??)...... but in any case, send him your old shoes for relining and upgrading the material.


2.  Brake squealing: This comes up rather often. All sorts of fixes are touted (including glues and pastes for the disc brake types on the back sides of the pads).  Some have used BMW-Brembo CAR type anti-squeal backing plates (dangerous on an Airhead with near stock thickness pads, particularly with any air bubbles in the system, which will expand and pressurize with heat).   Some 'fixes' will work for some time, as will, for a time,  changing to a soft material, disc changes, etc. While different models of our airheads are specified with different brake material, they are ALL designed to be used rather vigorously....specifically the design is for the worst case...moderately extreme conditions!  That is, BMW does not want your disc brakes to fail under relatively heavy usage on steep downhill mountain passes.    Pads tend to squeal from localized, usually small, areas of the pad where the surface has chemically and physically changed, let us just call it carbonized, whether or not you can see any visual change. 

The BMW Brembo CAR type anti-squeal plates are 34-21-1-116-006.  Be SUPER CAUTIOUS on using them!  I am very hesitant about even mentioning them here!  You MUST have a system with no air bubbles in it; and you MUST have pistons that retract properly, and you MUST have considerably WORN PADS to even think about using these.    These can be used on airheads ONLY if the pads are thinned in thickness first (either sanded or actually worn a fair amount)...otherwise they could seize.  NO air in the system...air expands with heat.    You do NOT want to be riding a bike with a sudden brake seizure!! These plates are particularly insidious for airheads if any bubbles of air are in the system which could expand when hot and cause the brakes to seize and lock.   So, if you intend to use those plates (NOT recommended by me), have well worn pads first, remember that I told you not to do it, and NOT to install them.

NOTE:  BMW has had bulletins about squealing.  They noted that the chemical structure of pads and shoes changes with age and temperature, and the material is designed for a very long life, INcluding time.   They are designed for high stability under EXTREME conditions. Extreme conditions, to me, means using lots of brakes at a goodly speed down a steep mountain pass, pulling a trailer, ...etc.    BMW noted that deceleration from lower speeds, such as city driving, does not allow the material to get as hot as would braking from higher speeds.  Thus, the pads become harder (at the near outer area), causing squealing.   YOU NEED to do some MODESTLY vigorous braking now and then to reduce, or eliminate squealing.  Speed to produce the right temperatures is important here, not just braking hard from 30mph which is not generally going to be the best thing to do!  In fact, you don't need to brake very hard. You need those linings to HEAT UP, from MEDIUM hard braking, maybe from 70 mph to near zero.   If you seldom use your brakes much, particularly enough to heat them up real well, the SURFACE of the pads (especially the outer area) will HARDEN. It is WORSE if your pads are dragging a bit. 

Squealing on disc brakes is the result of failure to use brakes reasonably vigorously, or having dragging pads.  Once in awhile it is pad surface problems, often in conjunction with dirty discs (and OFTEN the disc HOLES are grungy).  MOST of the time, NOT ALWAYS HOWEVER, if I hear a BMW near me that is squealing, I usually guess that the rider does not use his brakes much.  If you remove the pads, a very easy job, you can place them on an upturned piece of fine grit sandpaper (NO emery or other types!!) on a flat piece of glass...whatever.....and make even-pressured figure eight motions, and remove a SLIGHT amount of material. If the pad has grooves filled with crud, clean the crud out!  IT IS IMPORTANT that the disc HOLES be thoroughly cleaned out...use whatever means, including Q-tips, with a fast drying solvent (regular brake cleaner is NOT very strong)...whatever.....and note that just because the holes appear clean does not mean that they are.   If the holes are not cleaned, the crud in them will re-contaminate the pads.  Clean the surface of the disc, then holes, then disc, then holes, back and forth until no more black crud comes out.

NOTE:  If you change the type of material ...in fact, I recommend this for any pad change, even the original type of material,... you should not just clean the discs, but you should clean them up with maybe 500-600 grit SANDpaper (NEVER emery or other nasty papers), as some INVISIBLE old pad material may have been carbonized onto the disc surface....and then clean up with a good solvent.  

 I have had many questions about REAR brake squealing.   Here is some additional information from BMW, condensed for you.  This information was in TWO BMW bulletins, called SI's for their official name:  Service Information, and their identifications were:  
34-013-88 (2296)
34-011-86 (2230)

This information applies ONLY to the REAR DRUM brakes.  The application here is for any drum brake with shoes that are 25 mm wide (that is, 1 inch).  This means ALL rear drum brakes from 1985, and it also includes some older bikes, such as the R80G/S and the R80ST.  NOTE that all bikes FROM 1987 should already have these modifications.

According to BMW, one should take the approach in steps.  Do step #1, if squealing not eliminated, do step #2.
The first step, #1, is to install rubber dampeners. 
#34-21-1-457-602 is 55 mm long, and is ONLY for the K75 that has sand-cast shoes.
#34-21-1-457-572 is 70 mm long, and is for K75 with pressure-cast shoes, and the R bikes.
These rubber dampeners are installed ONLY on the REAR return spring.  The FLAT side of these is installed against the brake shoe and pivot cam; the slant side is to face left (towards wheel).

Step #2:  If squealing not stopped:
   Install later brake shoes, that have a revised place to hook the spring to...it increases tension.
The UPPER shoe is 34-21-1-242-401; the LOWER shoe is 34-21-2-242-402.
How do YOU KNOW if you already have these shoes?
There is a circular stamping, with the numerals 86 in the center.  It is unclear to me if even later shoes might not have later numerals in the stamping.

HINT!   It is important on drum brakes that the actuating cam contact the full width of the shoe at exactly the same time.   Use a thin feeler gauge, and SLOWLY bring the cam into just barely any contact with the shoe....and file the shoe METAL SLIGHTLY, to ensure that the full width contacts at the same time....this is, of course, done with the wheel/tire removed.  This gives better braking, and may reduce squealing too.   Squealing on drum models can be singing of the springs, hardened lining surfaces (same vigorous braking use recommended), or unequal/angular contacting of the brake shoe at the cam pivot....fixable with CAREFUL filing using thin feeler gauges and just enough actuation, to see if contact is EVEN, across the cam and shoe points.  Singing of springs can sometimes be fixed by stuffing them with a bit of rubber, or using the snap-on BMW rubber part.  Some use some spaghetti tubing over the springs.  


3.  Do NOT mix up the locations of pads, that will cause you to have to break in the pads to the disc again, and your braking will be poor for awhile.   This is particularly so with the swinging ATE brake calipers.

4.  ATE models of the swinging caliper type disc brakes have an adjustable cam at the bottom of the brake caliper.  This cam part must be cleaned and lightly greased and adjusted properly.  You can use an ink marking pen on the inside of the disc, and adjust the cam to give equal erasure as that brake is very lightly applied, wheel rotated.  If incorrect, braking is poor, and squealing more likely.  An article on the Airheads Club website, under Technical Tips, tells more about adjusting ATE swinging calipers.   http://www.airheads.org

     ATE Master Cylinders require an adjustment, which was done with a special U-shaped flat metal tool, that BMW provided in the owner's tools.  This tool sets the master cylinder (located UNDER the fuel tank for THESE ATE models) piston FOR THE FREE PLAY AT THE BARS LEVER.   The handlebars lever free play is to be 0.16" to 0.24", as measured where the lever end contacts the casting. When the lever is not under hand pressure, the lever end CONTACTS the casting; just to make this measured point clear in your mind.    To set this bar lever free-play, you must remove the fuel tank, and insert the special tool (pry off the rubber cover) into the master cylinder after loosening the cable adjuster locknut.  There is a groove in the MC piston for this purpose.  Adjust so the tool is JUST free to move, then tighten the locknut.   There is nothing OVERLY critical about this tool, but it should be flat, smooth, and the thickness fairly close to the original.  I measured an original one, in case you want to duplicate it:  The thickness of the metal is fairly important, the original one was 0.046"THICK.   The tool length is not important, the original was about 2".   The width of the tool was 0.592".  The slot in the tool's long end was to a depth of 0.642" with a full radius at the bottom; the slot width was 0.363".  It is certainly possible to do the adjustment without the tool.

5.  Use of "HH+" rated material will produce better braking, with less pressure needed at the handlebars, with the usual caveats on braking use.   A rather noticeable improvement in braking is had with Meonite cast-iron rotors and maybe a bit better braking from stock by installing full-floating SS discs, like those from EBC.  Those SS floating discs will not be all that much better, just a bit, than stock non-floating BMW SS rotors.    I do not recommend cast iron over the stock material, and it is not easy to find (one Canadian producer??), but it DOES work, but might be heavy, and due to the weight, affect handling.  The degree of  "better braking" with various changes "varies"!  Often a modification does NOT help braking.  Use of HH+ pads almost always helps,  and is likely the first thing to try, but be sure the manufacturer says that their material is OK for the BMW discs (ask not just about 'compatibility' but ask about WEAR!!).    Don't be stupid and install the new pads without first super-well cleaning the disc!!...that means the holes, the outer disc surfaces, and using sandpaper prior to the final solvent cleaning of the holes and flat surfaces.     Brake pad material should be compatible with your disc material.  If you use all BMW discs and pads, you don't have the problem of figuring it all out.   I have NOT YET seen any HH+ pads that are NOT OK with BMW discs or EBC discs.

NOTE!......if you change pads, remember that initial braking can be poor, until the pads break-in.  This is particularly so if going to a new type of pad containing, perhaps, copper or other metal.  This can happen with BMW's own pads too!    

Here I am, AGAIN, emphasizing this:    It is VERY helpful to clean the discs on both sides with a very fine grit paper, I use sandpaper or silicon carbide in 500 grit, and I am VERY particular about using Q-tips and solvent to clean each and every hole in the discs with a strong solvent, and cleaning the disc, and holes, over and over, before installing the new pads. For those of you that just will not do a perfect cleaning job, use sandpaper, not silicon carbide.    Do moderate stops for first 100 miles or so, and then the brakes should work OK.    If you fail to clean the small disc holes properly, the disc, which heats up in normal use, can release oily grunge, and contaminate the disc surface...and also contaminate your nice new pads!

6.   Single disc models can be converted to dual-disc, typically by changing a lower fork leg.   This CAN greatly increase braking power.   Once the system is fully bled and the lever feel is relatively hard and not spongy, and if the lever does not pull back too far...a TYPICAL situation, ....then there is NO need to change the master cylinder piston/bore size.   That stock smaller master cylinder size will produce more braking for a given squeeze on the lever, and this can be completely safe, IF...IF....the lever does not come back too far towards the grip.  AGAIN, only a properly bled brake system that has a nice hard lever feel will tell you this.   In every instance I have personally worked with, where I have added a second disc and used the same type and size of caliper, no master cylinder change of size was necessary....nor overly desirable.   Thus, if YOUR particular system works fine with the added second disc with the original size master cylinder, you have that extra advantage of that slightly smaller MC bore size for more braking (compared to what BMW used for DUAL discs), and no need to change the MC.   Keep in mind that putting another disc and carrier and a caliper on the front suspension will NOT improve handling!...rather, handling gets poorer!   This is due to the increased weight.   

NOTE:  BMW selected the master cylinder bore size on a particular bike for a number of reasons.   INcreased braking will come from using a smaller master cylinder bore, and some have done this even to an original dual-disc machine; such as changing to a single-disc MC.  Be cautious if you contemplate this sort of thing, you do NOT want that lever coming back all the way to the grip under ANY circumstances.   Be SURE that bleeding is done properly, before analyzing the amount the lever can come back.   The size of the master cylinder...the lever design for leverage, caliper size, etc., was all selected by BMW for a certain FEEL at the lever....one wants to be able to have a feel that is hard to describe, but you know it when you feel it (sorry for the pun).    In addition to that hard-to-describe feeling, you want to feel, via sensitive braking lever response, the wheel slowdown, before any locking-up (IF that were possible, typically not on an Airhead on dry pavement).   BUT, in every case I know of, the "feel" is fine with the extra disc added and the original master cylinder.  But, YOUR case may be different.  All disclaimers apply here!   ONE of the things I recommend, if converting from single to dual-disc, is that you do NOT go willy-nilly with a master cylinder larger bore size change.  Try the stock one, and properly bleed the system before making judgment!   IF you are replacing a master cylinder on a stock brake system due to its having failed and not being re-buildable due to its damage, typically deep pitting of the bore, you can consider a 1 mm , sometimes 2 mm, DEcrease in bore diameter....BMW has many different master cylinders available, especially for on-bars type.  Unfortunately, those with the G/S and ST, who have single discs, and who want more braking, have a limited selection of master cylinders from BMW, and may have to go aftermarket.

There are a number of places that can rebuild MC, resleeving, etc.: 
www.applehydraulics.com
www.brakecylinder.com
http://www.autosportseattle.com/
 

AND, see my REFERENCES article

7.  HINTS!!   There is an "accepted" left and a right side to front wheels, having dual discs.     Standard accepted assembly is for the nuts and the axle nut to be on the left side.  That means left as you sit on the bike.   Not  making the mistake of reversing the wheel will avoid caliper adjustments (ATE only) or having to break in the pads again (ATE or Brembo)  to conform to the fitment. It can also prevent you from having your front tire run the other way, as most are directional.  Mark your wheels.    Remember that tires are often marked for direction, via arrows on the carcass.  There are even front wheels so marked on the rim.....some discs also have arrows for forward direction.

8.  (a) A commonly done conversion is to convert an early under-gas-tank master cylinder Airhead to use the on-bars type of master cylinder.   What confuses folks is just how and what parts are needed.  Many folks do a conversion that is partial, but, for the truly interested, here is the complete list of parts, etc:
a.  34-32-241-567 "distributor" replaces the stock master cylinder, provides a hoses junction, and a place to screw-in the brake switch.
b.  34-32-1-241-565 is the upper brake hose for USA type bars....if you have low bars, you need the appropriate length of upper brake hose.
c.   32-72-2-310-747 is the 14 mm throttle and master cylinder assembly for years 1985 onwards....this allows the keeping of your stock dual throttle cables.
d.   34-32-1-242-205 banjo bolt, to connect the upper hose to the MC.
e.   two each 07-11-9-963-072 crush gaskets for the banjo bolt
f.    32-72-1-457-038 the cam gear for 32 mm carbs for the handle bar MC assembly
g.   51-16-1-237-641 RIGHT side mirror, with SHORT stem, for the USA bars, as the stock mirror will not fit the wider handlebar assembly.   For low bars, get the LONG mirror, or, whatever matches your current LEFT mirror, unless you are using bar-end mirrors, or?   If the bike being converted is such as an RT, which has the mirrors on the fairing, then this information does not apply to you.
h.  07-11-9-919-112, plug, for the right side of the 'distributor'.   This plug is NOT needed if you have dual-discs.
i.   If your throttle twist gear is worn, you will need 32-72-1-454-129
j.   If you want a new brake switch, it is 61-31-1-244-334
It is OK to obtain some of these items USED, but I'd suggest a NEW master cylinder assembly.

NOTE:  Brembo has always used hydraulic fittings threads that are 1.0 x 10 mm.

The stock turn signal switch will attach to the new throttle/MC assembly, and the stock lower brake hose attaches to the 'distributor'.     Use the stock BMW upper brake hose, don't use an uncovered SS braided hose.  It is possible to run a hose directly to the caliper, avoiding some parts, it is YOUR decision.

Twin-cable throttle assemblies from a 79-80 R65, or 1985+ models that have such, fit.  You CAN use a 1981-1984 throttle assembly, but then you need to convert the throttle cables to the one-into-two cables setup.  I differ with some folks on this, as I believe the one-into-two is a BETTER, more stable, throttle setup.  Master cylinders come in many bore sizes.   Smaller bores mean more lever movement but more braking for a given hand pressure.   Lever FEEL is also involved, and FEEL is not the same as pressure.   Generally, folks use 13 mm bore MC for single disc conversions, 14 mm for dual-disc.  I don't like the 15 mm all that much. Two different under-tank splitters are available.  One has three holes, one has four holes.    You can decide what you want to do, and with the brake switch in the lever....or in the splitter.

CONVERSIONS, 2-spot to 4 spot calipers:
When one contemplates a conversion, sometimes what comes to mind is to go even further than simply adding a disc or changing master cylinder bore sizes.   Maybe you are thinking of upgrading the braking on a later model that already has the MC on the right bar area.   Even with the stock on bars (or under fuel tank) master cylinder, adding more spots to the caliper(s) is a way to gain a VERY LARGE improvement in braking.
       One popular method is to use a BMW Oilhead caliper.  You can use the LEFT Oilhead Brembo caliper from the earlier Oilhead years, and it is then mounted to the RIGHT side of the airhead BREMBO mounts.  You need to mill about 4 mm off the mounting surface, and the larger piston then becomes the leading instead of the trailing piston, leading to weird wearing patterns, but a substantial braking improvement.  You can use the stock Oilhead brake pads.   This conversion is popular in Europe.   Detailed information is on the www.powerboxer.de website.     Swapping can be done for both sides, using the Left-Right, as described above, and reversing for the LEFT side of the airhead (that means using the RIGHT Oilhead caliper at the LEFT airhead side).     If I was doing this sort of conversion, I probably would try to get the two Oilhead calipers from such as a BMW salvage yard.     The earlier Oilheads calipers have identical bolt-hole spacing , and only need that approx. 4 mm milled off the mounting tabs.  The 4 pot caliper uses 32 and 34 mm pistons.     For those interested, the 2 pot units on the GS are 48 mm.     NOTE:  the 4 spot calipers from classic K bikes, as well as from Oilheads, will likely fit, with a little milling to the mounting tabs.  If the mounting hole center to center distance is the same as your Airhead mounting holes, this is going to be a relatively easy job.    The Airhead mounting holes of 108 mm spacing will accommodate these 4 spot calipers easily.

Whilst the dual-size calipers have some modest advantages, and the later calipers are stiffer....the major increase in actual braking comes from the increase in TOTAL caliper piston area.  Calculate from the area of a circle:  each piston has an area of its radius squared times pi.   That means that a 38 mm piston has about 1134 square millimeters of area...as one example.  Add up the total caliper piston area, and compare to the unmodified system total piston area.
 

The information in part 8. (b) below is partially useful for the above. 
The information in part 8. (c) is very useful for the above 4 spot caliper conversion.

8. (b) General discussion of adding a second disc, and effects of master cylinder size:
       When BMW installed the optional twin discs with the original ATE system, they went to a larger ~16 mm master cylinder.   BMW did the same thing when it dropped ATE and went to Brembo brakes and then
        went to twin discs on a single disc motorcycle.   I believe that BMW is overly-conservative in their selection of master cylinder sizes.  Note for reference, since you might be thinking of this, that BMW has almost
        always used the same leverage at the master-cylinder, meaning the leverage between the M/C plunger and handlebar lever, said lever being a teeter-totter around its pivot bolt ('fulcrum').   This means that comparing
        M/C sizes and effects is much easier, as there is no changing lever situation to take into account.   If you change the master-cylinder bore size you change the feel and effect on the brakes per hand pressure amount.
        Feel is hard to describe.  Variables include bleeding perfection; pad and disc irregularities, hoses condition and flex; temperature changes, etc.  I personally prefer upgrading the Airhead brakes to twin-disc with 4 spot
        calipers; over just adding one disc.  Your braking will improve by a BIG BUNCH.

  NOTE:   BMW offered dual conversion kits for various models.  Kits are likely obsolete, but, never hurts to check on this; and maybe if a
        kit has been on the shelf a long time you can make a deal.  I never bothered to write down all the kit numbers.   Again, I prefer the 4 spot conversion(s).

        NOTE that there are tradeoffs when using the stock original smaller master-cylinder and adding a disc, or larger surface area caliper(s), etc:
        (1) The handlebar lever will move further, as you have more fluid moving.  However, this is only a very small amount of fluid.
        (2) The 'feel' of the lever will change a small amount.  You might describe it as being slightly more spongy. This may be offset a fair amount to your liking, however, by your enhanced ability to modulate small amounts of brake force
        (3) You need to bleed the brake system rather carefully, as if you leave any bubbles in the system the lever will move backwards too far. 

8. (c)   Conversions of all sorts and types have been made to 'upgrade brakes.   See item 6 too.    One of the best ways to get increased brakes is to use the same make of caliper, but to use a
           caliper with more piston area.  Multiple spot brakes.  Oilhead and K bike brakes.    You can certainly convert to a Japanese bike multiple spot caliper(s), they are often cheaply available.
           I prefer to use the Brembo 4 spot calipers, as in the photo below or similar Brembo calipers, as has been noted above, the installation is likely to be rather easy, over-all.

IF you are converting the CALIPER SIZE, such as going to Classic K bike (or Oilhead) 4 spot brakes; I could, here, advise you that you cannot necessarily make assumptions
..that is... to follow my advice/discussion, above, in 8. (a) and (b).  I'd probably advise you to to either consult with someone who has already done the conversion you contemplate;
or, you can calculate the total increased piston diameter areas, compared to stock, and then get a good idea of any change possibly needed in the M/C bore. 
BUT:  Since it quite often will work out that the MC size is going to be OK, MY SUGGESTION is to not bother with 'working this out' in the beginning, but to FIRST
just try using the stock bore master cylinder.  It may work out fine!   Some conversions have been done and posted to adventure rider, with details.

Remember, MY INPUT is not any guarantee that your work is going to operate correctly, be safe, and all the other lawyer-ese things.   However, if your conversion is done neatly and
with good workmanship, the milling (if needed) done flatly and squarely, and everything bolts-up and lines-up properly and with NO free play in mounting bolt area, etc., then you are
likely going to be VERY pleased with the results.
Do NOT do this type of conversion without considering the risks if you do not do things properly.  I am NOT responsible for your poor judgement nor poor workmanship!

Below is a simple conversion, done to a R80ST, that incorporates a Brembo 4 spot brake.  Only a bit of milling was needed on the caliper
       in this 'easy' installtion:

       
In the above type of conversion, only a small amount of material was milled on the backside of the
mounting tabs of the caliper, and the caliper mounting centers distance was the same as the stock
distance on the fork lowers.   This makes for a VERY nice conversion.
Note that the conversion uses a floating disc, giving somewhat better performance.  EBC makes
floating disc assemblies, cheaper than BMW original non-floating parts.  If you had TWO of these
brake conversions on your front wheels, the increase in braking power would be quite large.
The only problem (besides matching component sizes and doing the installation) I have not mentioned
is that the WEIGHT of the front wheel will increase, which DEcreases handling somewhat.  You
probably will not notice.

8. (d)   Here are some references regarding master cylinder size, lever ratios, caliper sizes, etc.   I have given you some practical ideas in my brakes article that you are reading,
so take all this information as references for ideas....that is, perhaps those of you with a good dosage of nerdiness might want to see what others have written/published about these things.  
Some of this information I do not exactly agree with for BMW Airhead motorcycles.
Note that besides the below website articles, the Adventure Riders will have some conversions information from those who have done it.
http://www.vintagebrake.com/mastercylinder.htm    This is the information chart on MC and caliper combinations.

http://www.stoptech.com/tech_info/PedalSetup-DualMaster-Guide.pdf 

http://www.airheart-brakes.com/pdfs/MasterCylinder_Selection.pdf

When reading any of the above website's above, pay attention to the effect that lever (or pedal!) pivot point ratios can make.  There is
obviously a LOT involved in obtaining not just adequate or superior braking, but obtaining a FEEL TO THE LEVER that is acceptable or superior.
Pay attention to MY ADVICE, given earlier:  that I think you should TRY your old master cylinder size FIRST (be SURE bleeding is perfect!).

For many conversions, your stock MC is going to be OK.

 

You will notice that I have NOT given specific dimensions on how much to mill off your caliper(s); nor given you specific master cylinder conversion sizes (when appropriate), nor information on how and when and what to do about hoses and pipes, ETC.   This was done on purpose.    I am NOT going to be held legally responsible for any conversion you do.   My information in this brakes article is for your educational purposes; and that includes sections 8 (a); 8 (b); 8 (c) and 8 (d).    Brakes are of huge safety concern.  Failure to understand what you are doing; failure to do a proper installation; can lead to your death or injury.

I may, in the future, re-consider not supplying such information.

9.  It is MANDATORY, IN MY OPINION for the brake fluid to be changed/bled YEARLY, although some may get away with 2 years. This is for corrosion prevention AND prevention of moisture accumulation problems with lowering of boiling point. Seizing of brakes can throw you off the bike! If the fluid boils, and the boiling point is constantly lowering as the fluid gets old and more moisture is in it, you have ZERO brakes!!     If you put your bike up for the Winter, do this job during PRE-Winter servicing.    Do NOT use DOT5 SILICONE fluids.  Do NOT confuse 5 with 5.1 (I ALSO recommend not using 5.1).   For most folks DOT 3 is OK, although BMW recommends DOT 4.  Dot 4 is better, as it has a higher boiling point when it has some same amount of moisture in it.  If you are a very vigorous and heavy user of brakes; where the brakes get VERY hot,...this usually means big loads, steep mountain passes and VERY vigorous brake use, you could consider a premium DOT4 with a particularly high boiling point for additional safety factor.  CASTROL makes such a fluid.   BUT, be sure to change THAT fluid yearly if it is one of these extended temperature types. I recommend 6 months for that sort of race fluid.  On a practical basis, even with those who literally almost race down mountains with sidecars, I have NOT heard of common inexpensive DOT4, changed yearly, having ANY problems.   

Glycol brake fluids are 'hygroscopic' (note the g in that word), they absorb water....right through lines, microscopic places, MC screws/covers, etc.  Sunlight affects the reservoir fluid if the reservoir is on the bars on your model.    

 DO NOT use high pressure water spray while cleaning your motorcycle, where that spray can get to the master cylinder.   Failure to change your fluid, especially after it changes to light tan/straw color, will result in expensive repairs eventually, and might result in serious brake fading.  Carefully clean the inside bottom of the master cylinder reservoir, when doing a change/bleed.  I use Q-tips.   NO lint allowed in the master cylinder.  ONE fresh 8 ounce can is more than enough to do the bike (even if twin discs front and a disc rear).   Do NOT allow brake fluid to get on the paintwork!! as it will INSTANTLY destroy paint! (water and rag should be available right next to you as you bleed or otherwise work on the system).    I put wet rags around the master cylinder area to protect against accidental spills.   

 Do NOT use brake fluids specifically stating that they are for race use...UNLESS you change the fluid often.  NEVER use anything but brake fluid when doing simple on-bike cleaning of brake hydraulic system parts; or, use specific brake cleaning fluids. 

HINT:  When doing a fluid change and bleeding, and especially if the fluid is rather dark in color, obtain a baby-type of ear syringe and syringe out all the fluid from the master cylinder....and use clean Q-tips to clean any carbon looking matter out of the corners of the MC reservoir (careful!!...NO LINT allowed in the chamber or port at bottom!!!!!!!)...then immediately fill the master cylinder to about 1/2 to 3/4 with FRESH fluid, then start your bleeding.    Do NOT leave the m/c above the maximum fill line after you are all done.  It is better to have the level a bit lower than the maximum line.    If the rubber bellows has deteriorated, replace it.  Do NOT overtighten the top cover on the handlebars master cylinder....there is an updated (rectangular models) cover available....early rectangular ones warped.   When the improved rectangular cap/cover is used, a gasket under it was eliminated.  My old hand-scribbled notes said that the updated cover was 32-72-1-454-945.

10.  BMW stock-original type rubber hoses will last MANY DECADES, if you never hang the calipers by the hoses, which is true ABUSE!   Hanging calipers by the hoses ...will...or can... pinch/kink the tiny diameter hidden internal PLASTIC hose, especially at the hose end fittings.   It is RARE for the internals of a stock-original BMW hose to collapse, so long as the hose is not injured by that mentioned hanging.  Stainless steel lines are NOT all that much better...many are MUCH worse than the stock hoses, and many have even smaller inner tubing diameters   The problem with nearly all SS hoses is that they won't pass the WHIPPING test...a test of constant flexing to simulate long term use; and, the 304 SS material will work-harden, then can break; also the bad braiding area can rupture...and you get a bubble situation that can blow out.   SS hoses are NOT just SS braid covered standard rubber hydraulic line hoses.  If you use SS hoses be SURE that the OUTSIDES are covered by plastic tubing ...not just to protect painted surfaces, but to HELP avoid the common SS hoses/lines failure modes.  SOME premium hoses WILL pass whipping tests.  ASK! 

I know that this is probably controversial, but, except for racy appearance, I do NOT like SS braided  hoses.   Not only from the whipping test failures, but also because many of them have a TOO SMALL DIAMETER of internal plastic hose.  I think the STOCK hoses are BETTER, in nearly every way (exception:  Racy looks).    I will explain that too-small-diameter 'problem':  If the inner diameter of the inner plastic tubing is too small, then relatively rapid multiple-uses of the brake lever can build up UNrelieved pressure in the tube, and the brakes do NOT have TIME to fully release.    ALSO, the smaller the inner tube, the easier it is to get KINKED.     When a kink occurs, it closes-down the size of the tube.   For RETURNING fluid (to the master cylinder), that is BAD, as the returning fluid is under VERY LITTLE PRESSURE, and may not return!  When you pull on the lever, the pressure in the tubing is monstrously increased, which can easily force OPEN the 'kink', and allow the caliper pistons to move outward.  Upon lever release the pistons do NOT RETRACT FULLY.     This leads to high wear of the pads, overheated and warped discs, excessive deterioration of the fluid, ETC.

BMW hoses are not overly long, which tends to minimize the potentially softer feeling (from using non-metal-braided hoses).  ALSO, take NOTE here, that if the brakes are properly bled in the first place, that the FEEL of the lever should be such that the brake lever is NOT super-abruptly super-hard.  You would NOT like it that way, as you would find it MORE difficult to properly engage the exact amount of braking you want.
NOTE that BMW hoses are of the HIGHEST quality.    With a stock hose you can generally feel the hose to see if it is failing, especially near the end fittings, while you apply heavy lever pressure. That is not so easy with SS braiding, a drawback.   SS hoses require covering by clear plastic tubing of some sort, where the hoses might touch paintwork.   Some SS lines are far too stiff, not really meant for all the flexing that your airhead does with its moving fork.  Some (or most?) SS hoses have far too small an internal diameter of the inside tubing (often Teflon or?), and that creates a harder lever feel, but creates other problems as the lever pressure is released.   I like the LOOK of SS braid covered hoses, but not the potential problems.    

 Installing new hoses of any type MAY lead to a lot of cussing and a LOT OF LABOR....as you try to eliminate bubbles of air.   

NOTE:  as described earlier in this article, our hoses age over the years, although VERY SLOWLY if not abused.  Still, there may be a tendency, however small, but perhaps now increasing....for a hose to have an internal flap problem in which the hose allows pressure to go to the caliper, but does not allow (or only slowly) pressure to be relieved fully at lever release.    This will result in insufficient caliper piston release.  You can check for this by hand rotating the wheel and checking for anything more than lightest friction from the caliper pads.  Of course, the problem could be the piston and O-ring, corrosion, etc....as described earlier.

11.   BLEEDING:   Bleeding/changing fluid on brakes could be an entire article in itself.  Here is a method that works fine ASSUMING that you already have a fairly well bled system:
Attach a clear piece of plastic tubing that fits modestly snugly to the caliper bleed port, with a box-end wrench over the tubing, and have the tubing go UPward into a loop, then downward to a catch bottle.   Be sure the master cylinder is at least 1/2 full.  Be sure you have the area below the master cylinder covered by a fairly water-wet rag, and more water and rags standing by.  This is particularly important for the on-bars master cylinder.  You do NOT want, ever, the slightest amount of brake fluid to get on your paint...and the fuel tank is right there!    Squeeze the lever (hand or foot) and while KEEPING some pressure, loosen the bleed port JUST enough so that the lever moves maybe 2/3 of the way to the stop (at the most).  Avoid moving the lever farther, which it will, if the bleed port is left open too long or you opened it too much.  Before the lever moves all the way, tighten the bleed port lightly.  Yes, this is a two-handed operation if you do it by yourself.   Release the lever. Wait a few seconds.    Repeat the process over and over until the fresh fluid is clear and no bubbles appear in the clear plastic tubing.   DO NOT allow the bleed port to be open as lever is released, that can suck-in air, particularly at the beginning of the process before the tubing has an inch or two of fluid in it.   YOU should be observing the fluid coming out of the bleed port, looking at COLOR....and for tiny or any size bubbles.  Color will eventually change to new fresh clear fluid, and NO bubbles of air (no matter how tiny), as the system is bled.  Replenish the master cylinder BEFORE it gets empty...failure to do THAT will cause you enormous bleeding problems as you suck air and force air into the system!!  You do NOT need the rubber bellows, if you have one, in the master cylinder, during bleeding; but having the cover on, no need to hardly tighten the screws, is a good idea.

If you have opened the system, meaning removed a hose or fitting or worked on the internals of a caliper or master cylinder (NOT meaning just removing top cover from master cylinder), then the bleeding process can take a very long time, and in some instances a bubble of air can get trapped at some junction, and cause you to be VERY unhappy.   Pressure and vacuum methods are available.   In some instances, with the UNDER-tank master cylinder, you may have to move it to a different position to eliminate such a bubble of air.  On the rear disc brake, rotating the caliper for the bleed port vertical is a must if the system is very spongy and you have not been able to bleed it otherwise.  In fact, I recommend that even if the foot lever feels OK, that you do all rear bleeding with the bleeder port vertical.  That will ensure better bleeding.   Remember, bleeding cleans the system of old fluid, not just removes air bubbles.      To rotate the bleeder port vertical, without removing the caliper (and the need to insert some sort of 5 mm object between the pistons), you need to unfasten the holder's bolts. 

Bleeding problems can also apply to SOME front caliper(s) if they are really spongy, and the bleeder port not quite vertical.  

HINT:  SOME BMW airheads with twin front discs have ONE brake line leading to one caliper, and a crossover line to the other caliper.  If that is what you have, bleed the most distant caliper in the system, that is, the caliper farthest, line-wise, from the master cylinder, FIRST.  That is a good policy for ANY brake system....bleed the farthest distant caliper first.  

HINT:  If you are changing a hose, or otherwise 'opening' the system, you may have lots of problems trying to bleed the system properly.   Vacuum bleeding at the bleeder port may work, might not. There are PROBLEMS using vacuum devices, such as the popular MityVac.   A whole story unto themselves.  It is easy to see a connection constantly emit bubbles, when they are actually coming from OUTside, leaking into the bleeding fitting/hose.  You need to know all the tricks and hints when using a MityVac or similar device, whether pressure or vacuum bleeding method is being used.  Filling the hose, or any part upwards, as best possible, from the caliper bleed port, using a syringe/needle and clear tubing setup will help, as will tapping on every part to release bubbles, and letting the system sit overnight to allow microscopic bubbles to rise.  You can try putting a stiff rubber band on the lever, to keep pressure and lever position overnight, further allowing bubbles to rise.  Under some circumstances you may have to repeat this.  Don't use full lever-back position if the MC is an old used one....you can push the rubber seal into a crudded area. Pressure bleeding is an option. It does not always work.  With the UNDER TANK ATE type brakes master cylinder, you may have to move the master cylinder position in order to get out the last bubbles. In addition, the under-tank ATE type may have an air bubble at the SWITCH there.  You can fill the switch working end with fluid.  

 The REAR brake will require the bleed hole to be VERTICAL!!

Think THRICE before willy-nilly opening up a system to install fancy SS hoses!

FIRST:  Try normal bleeding, without vacuum or pressure equipment.  If doing the front brakes on a bike with the bars-mounted master cylinder, turn the fork to full LEFT, so the master cylinder is the highest thing in the system...that sometimes dislodges a bubble, but might take overnight to reach the reservoir.    Use the lever a number of times with the forks to the left, then let the bike sit overnight.  If that does not work, then  I recommend TYING the lever to about 3/4 way back, letting the system again sit overnight that way.  I use a block of wood shaped properly, to prevent the lever from coming full back (unless the master cylinder is new or newly rebuilt, then no block needed).  That does not let the bubbles come back into the master cylinder, but it pressurizes the system a wee bit (depending on situation), and then the bubbles will rise, you hope, and be easier  for them to get into the reservoir, when the lever is released.  So, if doing the overnight thing, release the lever in the morning, and let the lever alone!...for a couple of hours.

If this does not do the job, the next thing is the short jabs at the lever method, and tapping at the area of hose connections, at MC outlet, etc...then again let the bike sit overnight (don't tie the lever).      Keep in mind that tiny bubbles rise over time, and will rise to the highest level where they can be trapped by sharp turns and so on.   Another thing to understand is that if the substantial collected air bubble is at a very high point, that when you use the lever, the fluid may flow UNDERNEATH the bubble!  This is why the short jab may well dislodge the bubble and cause it to return to the MC, or, be expelled at the bleeder in some instances.   Be patient!

HINT!:    This happens mostly with the ATE under-fuel-tank master cylinder...but can happen with the on-bars MC.   You can't get pressure in the system after working on it.  You bleed and bleed, you try overnight waiting, you try vacuum methods, pressure methods...and still no lever pressure.  You wonder what is what, and your cussing vocabulary is expanding.  Well, the problem is very likely a bubble of air in the SWITCH at the master cylinder.  You will have to remove the switch, fill its cavity and reassemble in such a way that you try not to introduce any bubble.   

12.  Some squealing....and ESPECIALLY brake lever pulsing...will be had if the shoes do not contact the brake cam properly with drum brakes.  This can be fixed by using a feeler gauge and very slight brake lever movement...to see if the cam is contacting evenly across its surface, and if not, filing the shoe metal to match perfectly.  In addition, you can try filing down that point of the LOWER shoe by about .004".   If the bushing area is worn out, you will have to fix that.

13.  BMW uses different types of brake materials in various models.  It is up to you to make the decision regarding use of a different type of lining or pad material for your bike.   Note cautions earlier in this article.   Linings are rated by friction cold and friction hot (and a host of other things).  Letters are used to identify RELATIVE friction values.  "FG", for instance, has a lower friction (F) cold, more friction (G)  hot...compared to some unidentifiable standard.   The higher the letter, the higher the friction value.  SOME pad types are not compatible with BMW discs...but I have not seen that problem much in recent years.  NOTE that INcompatibility can take several forms.   Incompatibility can mean great brakes and high wear!   GENERALLY BMW has used FF and FG material.  HH material has very high friction. A popular lining is HH+.   BMW does not use HH.  It is acceptable for most folks though, and increases braking...but, keep in mind the various ramifications about changing such things, including faster disc wear (sometimes that happens).  It is important that the pads you are purchasing be rated to work with the disc material you are using...so ASK the seller or contact the maker.  It is sometimes difficult to get reliable information about disc compatibility and wear from the various linings.  Frankly, disc pads and discs last SO long, I am UNconcerned about life, but, of course, I am not buying you your discs.  NOTE that it only takes a very tiny bit of dirt, grain of sand, etc., to get into the lining or on the disc to start a groove and land type wear on the disc (pad too of course).  This is NOT a big problem, usually NO problem....which is contrary to what folks trying to sell you discs may state.  Certainly, discs with grooves and lands are going to take time to have the new pads break-in to them....and in some instances the discs do need resurfacing....but this is tricky, and needs to be done on a surface grinder, and could be rather expensive, and certainly takes material off the disc...and there is a limit to the disc thinness...stamped right on the assemblies.   Discs that look pretty bad can be fine.   A bigger problem (?) might be if the discs get too much thinner than the minimum BMW allows.  

     You should meticulously clean the disc holes once in awhile, and I personally clean the disc surface with a CLEAN rag and a strong evaporating solvent, rather often...usually after each washing.    HOWEVER....when time comes to replace the pads, be aware that modest braking in the first few hundred miles will enable the pad to better match your grooved/land disc. NEVER replace pads without cleaning the holes and sandpapering the disc with fine grit paper (and then cleaning!)!  Only in the worst instances should the disc be machined to eliminate these grooves.  The official minimum thickness is stamped on the assembly.    You are on your own if you exceed BMW guidelines (my standard disclaimer!).  The very best method of resurfacing a disc (if it HAS to be done)...perhaps to eliminate some PULSING in the lever (disc warping is not often fixed by resurfacing)....is to go to a QUALIFIED machine shop that has what is called a "Blanchard Grinder"....and remove a few thousandths from each side of the disc.  Mostly, I leave the discs alone!   BMW discs have an official wear limit.  The limit varies with the model.  Limits are part of the disc brake CARRIER casting, that is, the limit is cast into the surface, something like:   min 0.18.   The limit marking is on the INside surface of the CARRIER, near the outer edge.  You sometimes have to look closely for it.
***If you have a warped disc, I advise that you do NOT grind the disc, but FIRST try to use very thin shims at the carrier bolts, which may well remove enough of the warping.  Grinding sometimes does not work well, if the disc metal has taken on a 'memory' in the material itself!!

14.  There have been some instances of SINGLE (only) front disc models having loose bolts where the disc assembly bolts to the wheel hub.  Use 33-31-1-108-204 washers on BOTH sides of the bolt and torque to 22 Nm.   If it was quite loose, check the flange, etc. 

15.  PRE-1981 final drives use a wider brake shoe, and a narrower pivot pin.  Late shoes will NOT fit correctly on older drives.  You may need to remove a ridge to make things work if you substitute shoes, etc....that is, a late used drum on earlier drive.

***Another way of stating some of this:  If you install a 1981-1984 rear drum onto an earlier drum brake airhead, you need to change some parts, and you will need:
1 each 25 mm shoe  34-21-1-242-403
1 each 25 mm shoe  34-21-1-242-404
1 each cam 35-21-1-454-836
4 rubber O-rings 07-11-9-906-328.
BMW actually had a bulletin to that effect, brought about by BMW discontinuing selling rear drives for pre-1981 bikes ('use the 1981-1984...and these parts....).

Brake shoes:
Brake shoes must fit the drum size, or braking will be vastly lessened.
Brake shoes MUST be fitted so that they contact the CAM exactly and evenly.  This is done by having the wheel off, and SLOWLY moving the brake actuating arm until the cam nearly touches, and using a THIN feeler gauge to determine that the cam contacts 100% of the metal portion of the shoe at the same time.  File-down the shoe to get this condition.
If the shaft has too much play in its bushing, the braking will be poor....repair that bushing area.
Shoes: It appears that EBC and DP are simply rebranding the actual maker, which is Ferodo.
Shoes:  BMW shoes work find, but may need a bit of reworking to fit correctly, same for above three brands.
Brake shoes change chemically from age and from repeated heat-cool cycling.  If your brakes are awful, start with new fresh high friction linings.

16.    Caliper rebuilding hints: 
    BREMBO (and some of this is applicable to ATE):  Break the torque on the caliper joining bolts before  removing the caliper. Calipers are hard to hold in a vise, this will prevent scratching the calipers.  Use compressed air to CAREFULLY unseat the pistons toward one another before separating the caliper. I place a thin piece of metal or wood between the pistons, to keep them from flying out. Then remove that piece and use air to finish removing the pistons, while covering the piston with a rag. Problems with resealing the halves after reassembly may happen because the bolts were tightened so much that the aluminum caliper halves distorted slightly.  In fact the area around the holes for the bolts holding the calipers together will warp slightly, you cannot see it, but it happens, and the caliper would weep fluid if you did not do what is necessary first.  Also, separated halves might never join up again in exactly the same position; high and low spots don't match exactly, and thus leakage occurs. The fix is to resurface the halves on a surface plate with some 1000 grit Wet-or-Dry paper, using wetted paper, water is OK.     You can start with slightly coarser paper.  Best to use only brake fluid for final cleaning of the halves before reassembly, then wiping down with a lint-free cloth. If you use other, stronger liquids as solvents for cleaning, be sure you flush them away with brake fluid before reassembly.   The rebuild kit MAY contain an envelope of silicone brake assembly  grease. This is for the piston seal and caliper bores. Use the compound very sparingly.  I generally use brake fluid as the lubricant on re-assembly, of all calipers; but, if you have problems, try silicon brake assembly grease.  Sometimes the pistons are devils to install.  Be extra careful that the caliper groove is totally cleaned, and that the piston is smooth at the edges.    Some auto parts stores carry the brake assembly grease, if it did not come with your brake repair kit. Lubricate the new square ring on the piston on all sides before assembly. Threads should be clean and then lightly lubricated with brake fluid. Torque the bolts on the Brembo caliper to 22 Foot-pounds.  Once all is assembled and the caliper installed and system bled, you can finish the job by washing off hydraulic fluid drips from the caliper with plain water.

NOTE!    Some Brembo caliper rebuilding kits do not come with the small inside O-ring.   You need a 6 x 10 x 2 mm O-ring made of EPDM rubber.  BE SURE it is replaced.

17.  For those with the master cylinder on the bars, if you are replacing a 15 mm size, which is 32-72-2-302-370; you can consider, for somewhat more powerful braking, the 14 mm size, which is 32-72-2-310-785.  This type of recommendation works with most master cylinder sizes.   Watch out for excessive lever movement.   I am NOT responsible if you have a problem.

18.  Rebuilding master cylinders:  Sometimes it is not cost effective.  Here is one place that can hone them, etc...plenty more:  www.autosportseattle.com.   You can also rebuild them yourself, and you might want to check with EuroTech for parts.
You can contact Ted Porter at www.Beemershop.com
Also, try www.applehydraulics.com
              www.brakecylinder.com
              http://www.autosportseattle.com
See my references.htm page, under B (brakes)
       


ATE Master Cylinders require an adjustment, which was done with a special U-shaped flat metal tool, that BMW provided in the owner's tools.  This tool sets the master cylinder (located UNDER the fuel tank for THESE ATE models) piston FOR THE FREE PLAY AT THE BARS LEVER.   The handlebars lever free play isto be 0.16" to 0.24", as measured where the lever end contacts the casting. When the lever is not under hand pressure, the lever end CONTACTS the casting; just to make this measured point clear in your mind.    To set this bar lever free-play, you must remove the fuel tank, and insert the special tool (pry off the rubber cover) into the master cylinder after loosening the cable adjuster locknut.  There is a groove in the MC piston for this purpose.  Adjust so the tool is JUST free to move, then tighten the locknut.   There is nothing OVERLY critical about this tool, but it should be flat, smooth, and the thickness fairly close to the original.  I measured an original one, in case you want to duplicate it:  The thickness of the metal is fairly important, the original one was 0.046"THICK.   The tool length is not important, the original was about 2".   The width of the tool was 0.592".  The slot in the tool's long end was to a depth of 0.642" with a full radius at the bottom; the slot width was 0.363".  It is certainly possible to do the adjustment without the tool.


19.   PADS!
A LOT of misinformation is floating around about compatibility of brake discs, pad materials, etc.     Rather than
get into a very long article about it, here is what you need to know, in brief:
     a.  Sintered pads are made of various types of soft metals, with a bonding agent.   These types of materials began being
          offered around the mid-1980's.   They have the BEST friction coefficient and EXCELLENT wet weather
           performance.
          Often this type of pad is called HH compound.   These pads last a long time, are quiet, and the disc will usually last
          the longest.  I VASTLY prefer these pads.  They have almost no drawbacks, except maybe a tendency to grab a bit
          abruptly at first use....especially on some types of these pads.
     b.  Bi-metal, Organic, semi-metallic, etc:   Made of various types of fibers, with soft metals added.  They are usually
          cheaper than sintered pads; give a softer, perhaps smoother (especially at first application) engagement.  
     c.  ALL new pads require a break-in.    Some relatively easy stops at first, which matches up the pad and disc
          irregularities; then more aggressiveness as miles are accumulated.

20.  PERFORMANCE IMPROVEMENT:

Depending on use, both drum and disc brakes can be adequate.   For drum brakes, being sure that the cam contacts very evenly and that the drum diameter is in specification, and for racing cooling is important.  For drum and disc brakes, you could specify a higher friction type lining.  HH+ suggested.  For disc brakes, cleanliness of the holes and disc and a proper system operation is very important.   If you have only one front disc, change the other fork leg and add a second disc and caliper.  You MIGHT find that the original master cylinder piston size is OK, and better than the larger stock one used normally with twin discs.    The REAR drum brake is better than the rear DISC brake.  The rear ends swap, on twin shock bikes....with minor work.    For those with the master cylinder on the bars, if you are replacing a 15 mm size, such as 32-72-2-302-370; you can consider, for somewhat more powerful braking, the 14 mm size, which is 32-72-2-310-785....as one example.

***Quite a number of Airhead owners have 'upgraded' (downgraded?) their disc brake systems (some have even changed rear disc to drum, and VV):

(A) An additional disc where only one was stock on that year and model.
(B) Modified disc(s), such as some other than stock type of metal, perhaps floating type of disc/carrier, and other changes, including drilled or undrilled or grooved or not discs.  NOTE that drilled or slotted discs are not drilled or slotted primarily for sweeping off rain water, as is commonly thought.  The real purpose is to reduce the gas that is produced from the resin binders in older (especially) pads being heated.  The gas goes to the pad surface, and thereby makes a 'hydroplaning' type of layer.  There are two other purposes.  Drilled discs help INcrease the coefficient of friction.  The holes' edges provide those edges to promote 'bite'.  The latest high friction pads minimize the effect somewhat.  The other purpose of drilling (or slots or even shallow grooves) is to help remove brake disc debris...they have NOTHING MUCH to do with COOLING.
(C) Different master cylinder piston size.
(D) Different pad material
(F) Different caliper(s), such as more pistons, or staggered size pistons, etc.  I have seen a LOT of confusion over why an 'upgrade' brake caliper...to one with more than one piston, where approximately the same total pistons area as the stock one, has not improved braking.
Sometimes these modified systems have the unequal piston size calipers mounted to their Airhead such that the stock LARGER piston, normally being the exit section (tire rotating normal direction) is mounted backwards from what the manufacturer intended.  This, with the gassing, see item (2), results in less braking.   This type of problem can be somewhat minimized by being sure you are using drilled discs. Staggered size pistons calipers are staggered in size specifically to help with this gas problem.   Very modern motorcycles with the latest type of discs and calipers and especially pad compounds, are much less susceptible to this problem of gas-hydroplaning, and may not even have drilled discs.  Beware of using the wrong pads!
 

Revisions: 

12/20/2007:  Incorporate all previous revisions.  Completely revise the entire article, mostly for clarity.  Add more information
                      on rebuild kits.
01/15/2008:  hyperlink to 38E added
05/11/2008:  Edit entire article
12/28/2008:  Minor updates, for clarity.
01/17/2009:  add to #13 and add new item, #19
11/10/2009:  add hyperlinks to item #18.
12/15/2009:  swap 19/20, add 19 info on pads
12/31/2009:  add ATE tool information to item 18
01/23/2010:  Clarifications, cleanup
04/08/2010:  Clarifications
04/24/2010:  Remove bad link
05/22/2010:  change article number
10/12/2010:  Edit article for clarity
12/15/2010:  Phosgene cautions, and some other clarifications.
01/18/2011:  Add 21
03/06/2011:  Add ATE MC adjustment to #4, having removed that note from REFERENCES article.
03/13/2011:  Clean up article in various places and ways.  Rework #8 a bit.
06/07/2011:  Add a bit more information and photo, about conversions.
09/08/2011:  Add 8 (d)
10/17/2011:  Add considerably more to the conversions information; mostly to clarify things.
10/17/2011:  later in day, add another conversion photo view

© Copyright, 2011, R. Fleischer

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