Contents
- Parts List
- Remove Master Cylinder
- Remove the Master Cylinder Piston
- Disassemble Master Cylinder Piston
- Clean Up Master Cylinder Bore
- Painting the Master Cylinder and Caliper
- Install New Parts In Master Cylinder
- Assemble Master Cylinder
- Install Master Cylinder
- Brake Caliper Rebuild
- Install Caliper in Front Fork
- Install Brake Pads in Caliper
- Install Brake Lines
- Revisions
I have been working on two R75/6 bikes, one is my own 1975 R75/6 and the other a project I took on to wake up a 1976 R75/6 from its long slumber on a back porch under a tarp underneath a pile of outdoor furniture. Both bikes have the single disk ATE front brake. The 1976 bike has a build date of 09/1975 and an updated ATE disk brake system with a 40 mm caliper piston and a revision to the master cylinder and fluid reservoir with only one hole to mount the reservoir using a threaded sleeve. My 1975 bike has a 38 mm caliper pistonF and a two hole master cylinder; the second hole is where a metal strap clamps the plastic fluid reservoir on the cast iron master cylinder. The rebuild procedure is the same for both types of ATE disk brakes but some of the parts differ due to the different size master cylinder piston and number of holes in the brake fluid reservoir.
The emphasis in this write-up is on what I did to my 1975 R75/6. I include some information about the different parts used for rebuilding the later 40 mm caliper and how to identify a 40 mm caliper.
Master Cylinder & Fluid Reservoir Mount Under Gas Tank. Note the Evidence of Brake Fluid Leak.
Earlier Style Master Cylinder Fluid Reservoir-Two Holes With Mounting Bracket
Later Style Master Cylinder Reservoir-Single Hole With Threaded Sleeve
Parts List
There are rebuild kits available for both the master cylinder and the caliper. MAX BMW has a caliper rebuild kit that only has the rubber dust cover and caliper piston seal as well as a full kit that includes a new caliper piston. My local dealers do not offer the “seals only” caliper rebuild kits, so I think this kit is specific to MAX BMW. The piston that fits inside the caliper bore on both bikes was not damaged so I only need the caliper kit with the rubber parts.
I replaced the small o-ring (part# 34 11 1 233 120) that fits on the locating pin of the movable brake pad: the pin and o-ring fit into the hole on the front face of the caliper piston.
The master cylinder rebuild kit has all the rubber parts and all the metal parts–except for one “top hat”–and includes a new master cylinder piston. The metal “top hat” fits on the front end of the piston and I reused the existing one.
In the list below I show parts for the earlier (up to 1975) and later (1976 and on) ATE disk brakes. Pick the ones that apply to your year bike. I also show the Spiegler braided steel brake line I chose for my 1975 “refresh” bike and the stock BMW rubber line. I selected colors for the Spiegler braided steel line to compliment the new Smoke Silver paint I applied to my bike; smoke black outer plastic sheath over the steel braid with gold fittings. I also painted the lower steel brake line that goes between the caliper and the braided steel brake line with gold caliper paint.
The front brake cable adjuster on the lever arm that is attached to the master cylinder has a locking nut and mine was bent, so I replaced it (part# 34 72 1 232 516).
| Brake Parts | ||
| Part No. | Description | Qnty |
| 34 32 1 233 159 | GROMMET | 1 |
| 32 72 1 232 516 | KNURLED HEAD NUT | 1 |
| 34 31 1 234 924 | KIT BRAKE MASTER CYLINDER-D=14MM (to 09/75) (Depends on model year of your bike) |
1 |
| 34 31 1 234 926 | KIT BRAKE MASTER CYLINDER-D=14MM (from 09/75) (Depends on model year of your bike) |
1 |
| 07 11 9 963 073 | GASKET RING STOP LIGHT SWITCH – A10X13,5-AL | 1 |
| 34 11 2 301 358 | SET: REPAIR KIT, BRAKE PADS – TEXTAR T290 | 1 |
| 34 11 1 233 120 | O-RING (FOR MOVABLE BRAKE PAD) | 1 |
| 34 11 2 301 7051 | D38 ATE REPL SEAL KIT CALIPER (MAX BMW Part#. Depends on year of your bike) |
1 |
| 34 11 2 301 7091 | D40 ATE REPL SEAL KIT CALIPER (MAX BMW Part #. Depends on year of your bike) |
1 |
| S-BM0028 | Spiegler Brake Line R75/6 (Used on 1975 Bike) |
1 |
| 34 32 1 234 699 | BRAKE HOSE – L=680MM (Used on 1976 Bike) |
1 |
Remove Master Cylinder
The ATE master cylinder is mounted under the gas tank on the spine tube using a metal strap. The front brake light switch is mounted on the front of the master cylinder and has two wires attached.
The brake fluid reservoir is attached to the top of the master cylinder casting. On 1975 and earlier models, a bracket and screw are used to clamp the brake fluid reservoir to the master cylinder. On 1976 and later bikes, a threaded sleeve with an Allen head inside the hole of the sleeve is used. The previous pictures show the two different style fluid reservoirs and the different methods for mounting them.
I remove the reservoir and the brake light switch. Here is the earlier master cylinder with two holes. The smaller one is threaded for the mounting screw that holds the clamping strap. Both holes use an O-ring to seal the holes against the fluid reservoir. Both o-rings are included in the master cylinder rebuild kit so if you have the single hole version of the master cylinder, you won’t use the small o-ring..
Earler Two Hole Master Cylinder
Remove the Master Cylinder Piston
A C-clip holds the master cylinder piston inside the master cylinder. I use a pair of C-clip pliers to extract it. I push the piston into the casting to take the pressure off the C-clip so it is easier to remove.
Location of C-clip in Master Cylinder
C-clip Pliers with Small Pins
Master Cylinder Piston C-clip Removal
Master Cylinder Piston C-clip Removed
Here is the master cylinder piston assembly. The front of the cylinder is to the left. Not shown is the internal spring still inside the master cylinder bore.
Master Cylinder Piston Assembly Removed
Disassemble Master Cylinder Piston
NOTE:
In the following description, front refers to the front of the bike unless explicitly noted.
The rear of the master cylinder piston has a number of parts that have to be removed to get to the rubber seal. The front seal sits in a deep groove and has to be lifted out and over the front of the piston.
I use a screw driver to press the “top hat” off the piston.
Removing Top Hat From Master Cylinder Piston
When the top hat is removed, I remove a metal cap, the rubber seal with a metal pick and then a brass washer.
The rear end of the master cylinder piston has holes around the circumference of the boss the brass washer butts up against.
Master Cylinder Piston Rear End Parts In Order of Assembly (Left to Right)
Clean Up Master Cylinder Bore
The master cylinder bore needs to be smooth without pits or scratches. Mine has some minor discoloration. Brake fluid is hydroscopic so it absorbs water. It’s common for the bore to rust if the brake fluid isn’t changed according to the maintenance schedule. The 1976 master cylinder bore had a number of deep pits from sitting on the back porch for who knows how long and I had to replace it.
I use a suitable drill bit and wrap 600 grit wet/dry paper around it. I overlap the paper to get a snug fit inside the bore. I wrap the paper in the direction of the drills rotation so it will tend to tighten the paper. I use masking tape to secure the edge of the paper to the bit and a second piece to hold the wrapped layers tight on the bit.
Attaching 600 Grit Wet/Dry Sand Paper on 1/2″ Bit
Wrap Paper for Tight Fit in Master Cylinder Bore and Secure with Masking Tape
I hone the inside of the bore with the drill moving the bit up and down for 20 seconds and then inspect it. All I see is a mirror like shine which is what I want.
Insert Bit with Paper Into Master Cylinder Bore
Polished Master Cylinder Bore
Painting the Master Cylinder and Caliper
Both the master cylinder and caliper needed painting. The master cylinder had started to rust and there are a number of nicks and chips in the paint on the caliper. I have access to a bead blaster. I sealed up the openings and used glass bead to remove the old paint and rust from the master cylinder. Then I thoroughly cleaned it with hot soapy water and blew it out with compressed air.
I masked off the openings and used the old brake line to plug the master cylinder outlet port. I used a cylinder of masking tape inserted into the thread brake light switch hole and a suitable bolt wrapped in masking tape to seal the cylinder bore in the rear of the master cylinder.
Ready for Painting with Brake Caliper Paint
I use brake caliper paint and let it dry for a couple days before exposing the parts to brake fluid. The caliper paint is an epoxy based paint. I have to finish painting in one hour. After that, the instructions state you have to wait for seven days before you can recoat.
Install New Parts In Master Cylinder
Here are the parts in the ATE master cylinder rebuild kit.
Master Cylinder Rebuild Kit Parts
NOTE:
The rebuild kit comes with instructions and several diagrams. However, the diagrams are too small to see the proper part orientation. And there is no clear identification of which parts go where. I hope the detail I provide below will make it easy for you to get the right parts in the right place.
On the top row are the seals that go on the front and back end of the master cylinder piston. Under that are the O-rings that fit in the holes on the top of the master cylinder. If you have the single hole brake fluid reservoir you don’t use the smaller O-ring. Next is set of metal parts. Starting on the left is a small ring that I never found a use for, so I didn’t install it. Next is the brass washer, then the metal cup that sits on the top of the front gasket and a new C-clip. On the right side is the new master cylinder piston and rubber cover that fits over the brake cable linkage on the rear of the master cylinder.
Assembly is the reverse of disassembly 🙂 More specifically, I put the brass washer against the boss with the holes on the rear end of the piston.
Master Cylinder Piston-Front Face
Master Cylinder Piston Rear “Brass” Washer
The large rubber seal with the angled skirt and small grooves in the outer edge goes next.
Master Cylinder Piston Rear Seal
Master Cylinder Piston Rear Seal Orientation
The metal cup goes on next.
Master Cylinder Piston Rear Seal Retainer Cup Installed
Master Cylinder Piston Rear Seal and Metal Cup Installed
Finally, the “top hat” goes on with the brim of the hat against the base of the metal cap.
Master Cylinder Piston “Top Hat” Installed
This is how the parts look on the rear of the master cylinder piston.
Master Cylinder Piston with Rear Seal Assembly Installed
The front rubber seal slips over the front of the piston. I use a metal pick to ease it on.
Installing Seal on Front of Master Cylinder Piston
Here is a close up of the assembled piston showing which larger rubber seal goes where. The front of the piston is on the left.
New Master Cylinder Piston with New Rubber Seals Installed
Assemble Master Cylinder
The two O-rings (or one if it’s a single hole master cylinder) are installed on the top of the master cylinder. Then the brake fluid reservoir is attached to the master cylinder casting.
Installing Screw Inside Earlier Two Hole Reservoir
The return spring, piston and C-clip are installed into the rear of the master cylinder in that order. I use the C-clip pliers to insert the C-clip into the groove in the rear of the master cylinder. Then the round brass colored pin is installed between the brake cable arm and the hole in the back of the master cylinder piston assembly. I put a bit of wheel bearing grease on both ends of the pin and install it after I mount the master cylinder to the frame tube and am ready to install the front brake cable.
Two Hole Master Cylinder Parts
Master Cylinder Spring Inserted in Bore
Master Cylinder Piston Assembly Inserted into the Bore
Brass Colored Pin Inserted Into the Hole in the Rear of the Master Cylinder Piston Assembly
Master Cylinder Hinged Lever Arm Engaging Pin
Last, the front brake switch is installed in the front the master cylinder with it’s metal sealing gasket.
Brake Light Switch & Metal Gasket
Master Cylinder Assembly from Rear
Master Cylinder Assembly from Front Showing Brake Light Switch
Install Master Cylinder
The master cylinder is attached to the frame tube using the metal strap. The two wires for the brake lights attach to the brake light switch. It’s best to attach them before attaching the master cylinder to the frame as they end up inside the cavity between the steering stem and frame tubes.
Master Cylinder Installed on Frame Tube
Front Brake Light Switch Installed in Front of Master Cylinder
Brake Caliper Rebuild
The following diagrams are from MAX BMW Parts fiche. There are two rubber parts inside the caliper, a seal ring that goes around the caliper piston and a dust seal that fits in a large groove on the outside of the piston. The dust seal is part number “8” in the diagram below. The MAX parts diagrams don’t show the piston that fits inside the caliper nor the rubber piston seal that goes inside the caliper bore. These parts are included in various rebuild kits.
R75/6 Caliper Parts Detail-Source: MAX BMW
R75/6 Caliper Seal Kit – Source: MAX BMW
NOTE:
The number in ( ) in the text below refer to the numbers in the MAX parts diagram above.
Different Size Calipers
Starting with the 1976 model year (09/1975) the ATE caliper piston size was increased from 38 mm to 40 mm. The caliper seal is different based on which size caliper your bike has. The 40 mm caliper has “40” cast into the outside of the housing while the 38 mm caliper has no marking at all.
40 MM Caliper & “40” Casting Mark
Removing the Caliper
I remove the caliper by removing the brake line and then the cover plug (5) and spring (4) that fits over the pivot pin (3). The cover plug is on the bottom of the left fork tube.
Bottom of Left Fork Showing Cover Bolt for Brake Caliper Eccentric Pin
Under the plug is a spring and then the eccentric pin. Sometimes the pin is stuck due to hardened grease. I use a magnet and a sharp pick inside the slot in the head of the eccentric pin to get it to drop down while rotating the caliper back and forth.
I remove the brake pads from the caliper. One pad is fixed to the caliper using a “hair pin” retaining clip. The other pad has a pin that inserts into a hole the face of caliper piston and I use a screw drive to pry it out of the piston.
Fixed Pad with Wire Retaining Clip
Movable Pad with Flat Face Forward Slides Out Of Caliper Piston
Here is the caliper when it’s removed from the bike showing the caliper piston front face with the center ring the caliper pin slides into and the rubber dust seal that fits between the caliper body and this piston.
Front Face of Piston with Center Hole Surrounded by Rubber Dust Seal
Disassemble Caliper
The dust seal fits into a groove in the caliper body and a groove in the piston. This seal was stuck to the caliper so I couldn’t remove it. Usually you can remove it before removing the piston. I use compressed air applied to the brake line hole to blow the piston out of the caliper body. I put a rag inside the caliper next to the piston to cushion the piston so it isn’t damaged, and I wear eye protection and stand to the side before I shoot air into the caliper. I have a rubber tip on the end of the air nozzle to help seal the air chuck against the caliper. Only a short burst of air is necessary.
Rag to Catch Piston
A Blast of Compressed Air Removes Piston
The dust cover was still stuck in the groove in the caliper after I removed the piston. It shredded as I peeled it off the caliper leaving small bits of rubber in the caliper’s groove.
Piston Free of Caliper But Dust Seal Still Stuck to Caliper Body
Old Dust Seal Shredded When Removed
The dust seal has a metal ring that slides inside the caliper bore to secure it. It didn’t come out when I pulled the dust cover out. You can see in inside the white circle and the arrow in the picture below.
Dust Seal Metal Ring Left in Caliper Bore
I use a metal pick to push it up and out of the bore.
Use A Pick To Push Dust Seal Metal Ring Out of Caliper Bore
Dust Seal Metal Ring That Separated From the Dust Seal
If it is still in the caliper bore, you won’t be able to install the new dust seal tightly in the caliper bore.
I used a pick to pull out the rubber seal that seals the piston inside the caliper bore.
Use A Pick To Remove The Caliper Piston Seal Ring
Piston Sealing Gasket Removed
The piston is hollow in the back where the brake fluid presses on the piston.
Brake Fluid Side of Piston
The front of the piston has an centered hole in the face of the piston and a large groove near the front for the dust seal.
How Dust Seal Fits Into Large Groove Near Front Face of Caliper Piston
Front of Piston Showing Hole In the Center for Movable Brake Pad Pin & O-ring
Inspect the piston for nicks and rust. If you find these, you will need to replace the piston.
Install New Piston Rubber Seal
I use clean DOT 3 brake fluid to clean out the piston bore of the caliper and the brake line and bleed line holes. I DO NOT USE BRAKE CLEANER. Brake fluid is all I want touching the inside of the caliper and the rubber piston seal and dust seal. I also soak the piston rubber seal dust seal in clean brake fluid before installing them in the caliper.
Soaking Piston Seal and Piston Dust Cover in DOT-3 Brake Fluid
The piston seal fits inside a groove in the caliper body.
Inserting Piston Seal Into Groove in Caliper Bore
The dust cap fits into the large groove on the front of the piston and into the small groove in the body of the caliper.
Dust Seal Soaked in DOT-3 Brake Fluid
Dust Seal Installed in Large Groove in Piston
Before I insert the piston with the dust seal into the bore of the caliper, I put a bit of brake fluid on the outside of the piston to help it pass through the rubber piston seal. It’s a tight fit and I have to carefully align the piston in the caliper so it is parallel to the caliper bore. There is a chamfer at the beginning edge of the bore in the caliper that helps get the piston aligned. I press the piston into the caliper by moving around the perimeter in a rocking motion as it slides into the bore. Eventually it slides all the way into the caliper.
Piston with Dust Seal Installed in Caliper Bore
I check to see that I can move the piston up and down a little bit using a pair of needle nose pliers to tug on the center hole of the piston being careful not gouge it. If I can’t then something is wrong with either the caliper body, the piston or the rubber seal around the piston. My piston moves a bit in and out of the caliper bore.
Install Caliper in Front Fork
When I have the front forks mounted in the bike, I install the caliper in the forks. The eccentric pin goes through the hole in the left fork lower to mount the caliper in the fork. It has a rubber o-ring that fits into a groove near the head of the pin. I use a pick to install the new o-ring as the old one was hard.
Brake Caliper Eccentric Pin & New O-ring
Pulling O-ring On Eccentric Pin With Pick
Install Brake Pads in Caliper
I have new brake pads to install in the caliper. One pad is fixed against the fingers on the left side of the caliper as you face the opening in the caliper and the other goes on the right against the face of the piston so it gets pushed against the disk rotor.
The back of the pads are different. The fixed one has locating bosses and a center pin while the movable one only has a center pin.
Fixed Pad with Raised Locating Bosses
Movable Pad With Only The Center Pin
The fixed pad is secured with a “hair pin” clip while movable pad stays in the hole in the face of the piston using a small o-ring.
Fixed Pad “Hair Pin” Clip & Movable Pad O-ring
Movable Pad with O-ring Installed on Center Pin
The pads have a flat end and a rounded end. The flat end faces to the front of the bike. I insert the pin of the movable pad with the o-ring into the center hole of the piston.
Movable Pad with Flat Face of Pad Pointing to the Front
The fixed pad is secured to the caliper body using a “hair pin” shaped clip that slips under the head of center pin as shown below.
Caliper & Pads Installed
Fixed Pad Secure with Wire Clip
Install Brake Lines
I bought a new Spiegler brake line with braided steel and custom color sheath and hardware to compliment the Smoke Silver paint on this bike.
Spiegler Brake Line-Smoke with Gold Fittings
One end threads into the hole on the left side of the master cylinder. The other end has a fitting to screw the steel brake line from the caliper into.
Braided Steel Brake Line Installed in Master Cylinder
Orientation of Brake Line Brackets
The line routes through the grommet of the bracket that attaches to the pivot pin assembly for the steering damper. This bracket mounts to the outside of the mounting lug for the front steering damper. The steering damper pivot pin nut secures the bracket.
Brake Line Routing Through Frame Bracket and Grommet
The second bracket attaches to the front fork. The bracket goes around the fork brace so the bracket strap is on the inside of the brace next to the tire.
Brake Line Bracket Location on Inside of Fork Brace
The other stap of the bracket fits under the reflector bracket on the outside of the fork tube. The fitting on the end of the flexible brake line has a threaded bushing on the end and the bushing fits inside a rubber grommet that fits into the large hole in the lower brake line bracket.
Brake Line Bracket Location on Front Side of Fork Tube
Install Lower Steel Brake Pipe
The lower metal brake pipe screws into the end of the flexible brake line and into the body of the caliper. I decided to paint mine gold using gold caliper paint to compliment the gold pin stripe used with the smoke silver paint.
Caliper With Painted Metal Line Installed
I’ll bleed the brakes and adjust the eccentric pin so the pads are parallel to the disk rotor when I get the rest of the bike assembled.
Revisions
2019-02-06 Allowed comments.
Thats a great write up Brook, pics make the story very clear. Thankyou.
Me, in Queensland, Australia…
My challenge… 1975 R75/6, 86000 km, recently bought and mechanically runs well, but I have had to rebuild the forks, bent staunchions, bump rubbers gone to mush, usual stuff.
And the front twin discs very sluggish with a jammed up master cylinder…
So, rebuilt that down from 17mm to 14mm on recommendation from a bmw lifetime mechanic. For better action with twin ATE calipers.
Fitted a new seal kit, pistons ok condition. The original piston seals that came out were tapered on the outside edge, I presumed from wear.
But, after fitting the new Parallel sided seals, no way can I get a hard lever feel, just mushy.
Bled by experts but no result.
Today I isolated the master cylinder by plugging off both ports, have a hard lever so the m/c is great.
Refitted the old tapered seal to one side, good feel at the lever, but the other side just weeped so no luck there.
My question relates to the tapered piston seals… are these original and available, or just tapered seals that have worn a lot.
Thanks again, all your write-ups here are invaluable… don’t stop.
Cheers, Gray Kay.
Hi Graham,
The rear master cylinder seal is tapered. The front master cylinder seal is not.
You don’t mention the condition of the calipers.
– If the seal there is bad, it can leak.
– If the bore and/or piston is rusted, it can leak.
If the brake lines are old and the internal plastic tube is cracked, the lines will bulge and you can’t build up pressure.
The adjustment of the lever arm of the under tank master cylinder also has to be correct for proper pressure .
You can get an air bubble where the front brake light switch attaches to the master cylinder body. A trick is to remove the master cylinder from the frame and tilt it so the switch is pointing down a bit. Then use a bungee cord to pull the front brake lever about 1/2 way to the handlebar. Leave it this way over night. Any small bubble will float to the top, and it only takes a small bubble to get mushy brakes.
I hope that helps.
Best.
Brook.
Hi Brook
Thanks for your reply, great info as always.
Do you remember which model has the rear disc with tapered seals..?
Calipers are good, pistons almost perfect.
Brand new brake lines.
Lever arm on m/c adjusted by the book, almost 2mm feeler into the groove on the outside end.
A gentle tap on the hand lever will see some liquid spurting into the reservoir, returning ok.
Brake light switch… we up-ended the m/c and bled a small amount of air from behind the switch, and then tilted the other way and left overnight with the hand lever bungeed up.
Good feel in the morning.
So, would I be better with new tapered seals from a rear caliper model, or try a different brand new square seal?
Thanks again, I have sent links to various parts of your rebuilds to friends here, all are very happy to have them,
Cheers again,Gray.
Hi Graham,
I have always used the master cylinder rebuild kit I get from BMW. I have relied on them to provide the correct design seals for the year/model bike.
It sounds like the brake light switch had the air bubble. 🙂
Thanks for connecting others with these materials.
Best.
Brook.
Hi Brook
Thanks, parts from Moto Bins… should be ok.
Do you know which model has the tapered seal for 40mm caliper?
Cheers Gray.
Hi Graham,
No, I don’t.
Best.
Brook.
Hi Brook, I have a ’75 R90/6 which needs the front brake m/c rebuilt. I’ve pulled it from the bike to inspect, no problem. Do you recommend a company that rebuilds the M/C? I’m not sure I have the skills necessary to do the procedure you described above, especially honing the cylinder.
Hi Opie,
I’m sorry, but I don’t have a recommendation. If you are a member of the Airheads Beemer Club, and you are in the US or Canada, then I suggest you contact your local “Air Marshal” to locate someone who can help you.
–> http://www.airheads.org/
Best.
Brook.
I used Apple Hydraulics for a rebuilt ($175). :[email protected]
Really good info. I have recently overhauled and replaced pads on my R100S. Am unsure on how to align the calipers/pads to ensure they are parallel with the disc. Any thoughts on this?
Hi Nigel,
The large bolt on the bottom of the caliper is removed along with the spring inside. There is an eccentric pin inside that hole with a large slot on its face. You turn that eccentric until the fixed pad is parallel to the disk. You can test this by putting radial lines on the inside face of the rotor. Then spin the tire and pull the front brake enough to get a bit of pressure on the disk, but so you can still rotate the tire. Spin the tire several times to see if the inside pad is removing the line evenly along it’s entire length. If it removed more at the top or the bottom, adjust the eccentric pin to make the removal more even. When you have the fixed pad aligned, install the spring and tighten up the nut.
I hope that helps.
Best.
Brook.
A guide made really well, congratulations.
I have to overhaul the master cylinder of a 1973 3/9 with kit 34 31 1 234 924 which is difficult to find in Europe, while kit 34 31 1 234 926 is available.
What are the differences? can I also use 34 31 1 234 926?
Thanks so much,
regards
Fabio
Hi Fabio,
Kit “924” is ONLY for single rotor bikes while kit “926” is for dual rotor bikes. That should make it clear which you need to order.
I hope that helps.
Best.
Brook.
Hi All,
Just stumpled across Repair set brake caliper with piston 40mm , ATE
PISTON MADE OD PLASTIC – No more rust – from BMW Bayer in Germany
https://www.bmwbayer.de/en/repair-set-brake-caliper-40mm-piston-made-of-plastic-ate-bmw-r2v-boxer-models-09-80
– any comments?
Hi Walter,
That’s interesting. I have no experience with this, so I can’t comment on suitability. That said, Brembo does not make their caliper pistons our of steel, but it’s not plastic AFAIK.
Best.
Brook.
Thank you for your through explanation. Please assist! The eccentric pin on my 1974 R75/6 is totally stuck in the caliper. Using an m8 bolt ive tried hammering on the thing, using penetrating oil and it will not budge! Im losing patience and considering sawing off the exposed part of the pin to free the caliper and hammer it out and replace. Any suggestions? I’ve been stuck on this for days and cant do anything else till I get it removed. Thank You! – Josh
Hi Josh,
I would try heat as well as penetrating oil. Heat the caliper body until it’s too hot to touch. Apply penetrating oil to the shaft of the eccentric pin and let it cool down. This will pull the oil into the down into the space between the pin and the caliper body. You may need to repeat this several times before the pin will start to move.
I hope this helps.
Best.
Brook.
Hi Brook…am restoring exactly the same machine and have exactly the same problem….only worse. Some dipstick in the past has inserted a bolt into the bottom of the pin and then very kindly snapped it off! So am left with 15mm of unthreaded bolt protruding. As you can imagine this is very little to get hold of. Now on third day of trying to remove this damn pin without totalling the bottom of the forks. Am coming to the conclusion that the only way round this is to cut through the pin and buy a new one. In England these are like hens teeth. Any other thoughts gratefully received. Phil from Pensford
Hi Phil,
I would try an easy-out (aks, bolt extractor) to see if you can spin the shank of the bolt and perhaps persuade the bolt+pin to back out. It may help the pin to come loose if you heat the caliper around the pin and also try some penetrating oil around the pin to help it budge.
Hope that helps.
Best.
Brook.
Hi Brook, thanks for suggestions. Have heated the parts to little effect…quite worried about how hot to get the calliper. The pin is well and truly welded to the
cast iron and probably has been for a considerable time..ie 30 years. What do you think about using a freeze kit instead. Getting to the stage where I really don’t want this thing to beat me!! Regards Phil
Hi Phil,
Well, best of success with this problem. When gentle methods fail, it’s time to get a bigger hammer I guess. 🙂
Best.
Brook.
I’ve just discovered this site while researching master cylinder repair kits for my R75/6. Thank you Brook for an exceptionally clear and informative write-up. If only all car and motorcycle repair sites were as thorough as yours. Very much appreciated.
Kit
Hi Kit,
Thank you for the kind words. It’s nice to know the information I publish is being used.
Best.
Brook.
Brook, I had the MC of my R90/6 rebuilt and don’t remember how the 2 sets of connections (brake light and fluid level) are supposed to be done. Thanks.
Chris
Hi Christopher,
I believe the front and rear brake light switches have GREEN-Black and GREEN-Red wires going to them. The fluid level switch has BROWN-Blue and BROWN wires going to it.
I hope that helps.
Best.
Brook.
Sorry, I wasn’t clear. I’m unsure about the left/right side connections for each of them.
Chris,
These are switches, so it makes no difference.
Best.
Brook.
Hello Brook,
First let me say thank you for your excellent documentation work. I don’t know how you find the time. You have a rare gift of mechanical skills, writing skills and photographic talent – and patience!
My question: My 1974 R75/6 has the original 38mm ATE caliper and 14mm under tank master cylinder. Front brakes are marginal at best even with new pads and careful adjustment. (Installing a new rotor soon.) I want to try re-sleeving the master cylinder to 13 mm or 1/2″ diameter to get more pressure at the caliper. The three shops I’ve talked to can handle the re-sleeving no problem, but I have to send them the piston I want to use. (Insurance liability.) With your experience, do you have any suggestions on a piston that would work with this master cylinder? In addition to the diameter, it has to have the correct length and seals in the right places.
Best regards,
Dennis
Hi Dennis,
Thank you for your kind words.
I have not attempted to modify the ATE master cylinder, so I have no information I can share with you.
Others have replaced the original 38 mm caliper with the later 40 mm one to improve braking, albeit modestly. The other major upgrade is to replace the under tank ATE master cylinder with the handlebar master cylinder introduced in the 1981 model year. However, that requires a complete change of the handlebar casting, throttle grip and throttle cables, so it is a major lift for the 1974 model that used the /5 handlebar casting.
You might contact Tom Cutter, at rubber chicken racing garage (www.rubberchickenracinggarage.com) for his advice on how to improve the brakes on your bike. He has been working on the airheads for close to 50 years and is one the acknowledged guru-class mechanics.
I hope this helps.
Best.
Brook.
Dear Brook,
Excellent and useful series of blogs, for which, thank-you.
My ’78 R100RS has the 40mm ATE front calipers, which I am rebuilding. The top seal (31 42 1 231 483) on the eccentric pin is rock hard and damaged so must be replaced. After 40+ years in place (afaik) it is very much at home and refuses to be evicted. As you know, it fits in a blind drilling in the fork leg, so I cannot get behind it, and is pretty slim, so there’s little on which get any sort of purchase.
Can you advise, please, how best to remove it, without damaging the drilling?
Thanks in anticipation,
Andrew Woods
Hi Andrew,
I would thread a bolt into the bottom of the eccentric pin as I show in the 1977 RS rebuild.
–> 34 BMW 1977 R100RS Rebuild and Restore Front Disk Brakes & Master Cylinder
You may also want to try some Kroil, or other penetrating solvent, for removing rusty bolts. Put some where the caliper bushing meets the casting of the fork slider and rotate the caliper a bit to help it penetrate into the gap around the pin. You may need to do this several times and let it soak in over night to help get the pin to move. And, you can also try heating the caliper casting (heat gun will work) before adding the Kroil. As the casting cools, it will suck the Kroil into the gap between the pin and the caliper
You can also try using a vice grip on the 8 mm bolt you threaded into the bottom of the eccentric pin and try tapping downward on the vice grip with a hammer while you rotate the caliper to get the pin to start moving.
I hope this helps.
Best.
Brook.
Dear Brook,
Very sorry but I did not explain clearly.
I have removed the eccentric pin (item 3 in your diagram) and the ‘O’ ring seal at the bottom (item 2 in your diagram, which sits in the groove in the larger diameter at the base of item 3), no problem. I used exactly the method you described, with a M8 bolt – but no force was necessary.
The seal in question sits in the fork leg slider and supports/seals the top of the eccentric pin (on the smaller diameter). It remains in place when the eccentric pin and the ‘O’ ring seal (items 3 and 2, respectively) are removed .
This is a ‘proper’ lipped oil seal, with metal backing – but quite fine. Part number as above but I cannot copy the reqd image.
Hope this is clearer.
Yours,
Andrew Woods
Hi Andrew,
Okay. I would try a curved pick to get behind the hardened o-ring and pry it out. I use these from Harbor Freight, but you can find them elsewhere I’m sure.
–> https://www.harborfreight.com/6-piece-pick-set-93514.html
Best.
Brook.
Brook, great article. 75′ 90/6. Trouble removing brake cable from m/c. Advice?
Hi Robert,
I’d try to pry the feral out of the curved fingers with a screw driver if that’s what’s stuck. You may also have to screw the adjustor threaded insert all the way into the MC housing to get enough slack to remove the cable. This may require removing the other end of the cable from the handlebar lever to get more slack.
I hope this helps.
Best.
Brook.
thanks and that is exactly what I did and it fell out..thanks again
Thanks for the detailed information Brooke. I am finished dis-assembling my ‘75 R90/6 M/C and am preparing to hone the inside of the cylinder bore. It is pretty badly pitted. I am hoping that honing will repair it and that I won’t have to have a sleeve inserted 😬. We shall see…
My bike is from 01/75. As a point of interest, and for those with older bikes, it seems that the little washer / oring that you found no use for was located between the plastic reservoir and the bracket that the small bolt secures to hold the reservoir onto the M/C.
As always your detailed approach and numerous photographs are greatly appreciated.
Many thanks (again)
Travis Chalmers
‘75 R90/6
Hi Brook,
Thanks for the detailed explanation, it helped a lot a beginner like me.
I haven’t seen how to place the dust seal metal ring when reassembling the cliper piston. Is it even necessary?
Thanks,
Ziv
R75/6
Ziv,
I believe the metal ring is integrated into the dust seal. When I removed the old dust seal the metal ring separated from the old seal, so I had to remove it separately. So, you don’t install the old metal ring when you install a new dust seal.
Best.
Brook.
Hi Brook,
Just found and finished reading this article.
Nicely written and very good progression photos, however I note no comment on the spring, actually nobody comments on the spring, and nobody stocks it,yet it is such an important part. I recently had issues with 1974 BMW R90/6 sticking on the disc. Stripping the under tank master cylinder found a faulty spring. I cannot re use this and I cannot find a replacement or dimensions after sending email around the world. I am hoping that you may be able to help by assisting with contact/s to purchase or helping with the diameter and length so that I can source one.
Regards
Brian Wilson
Hi Brian,
I’m sorry, but I don’t have those details. Perhaps you can find a used ATE master cylinder on Ebay, etc., for a reasonable price. You can also contact Duncan’s Beemers as he sources a fair quantity of used airhead parts. (http://www.duncansbeemers.com/partt2.html)
Best.
Brook.
“The rebuild kit comes with instructions and several diagrams. However, the diagrams are too small to see the proper part orientation. And there is no clear identification of which parts go where.” Yes, exactly. Thanks Brook for the detailed step-by-step and all the photos.
Hi Brook, Thank-you for brilliant step by step visuals!
This has been a great help for me to trust myself rebuild brakes an a 90/6.
Could you help me with the torques or elbow strenght values for all the connections:
1. the braided brake line nut to the mastercylinder port
2. the steel brake line olive nut to braided line
3. steel brake line olive to caliper
4. bleed nut on the caliper
5. brake switch to m/c
Many thanks – looking forward to further projects with your instructiions
John
John,
I just hand tighten them until they feel snug and tight. That said, I judge from the size of the nut/bolt what seems reasonable: smaller fittings get much less force. I don’t know how else to explain it.
Best.
Brook.