34 BMW 1975 R75/6 Rebuild Master Cylinder & Disk Caliper

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

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

Earlier Style Master Cylinder Fluid Reservoir-Two Holes With Mounting Bracket

Later Style Master Cylinder Reservoir-Single Hole With Threaded Sleeve

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

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

Location of C-clip in Master Cylinder

C-clip Pliers with Small Pins

C-clip Pliers with Small Pins

Master Cylinder Piston C-clip Removal

Master Cylinder Piston C-clip Removal

Master Cylinder Piston C-clip Removed

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

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

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 Front Parts In Order of Assembly (Left to Right)

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

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

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

Insert Bit with Paper Into Master Cylinder Bore

Polished 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

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

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-Front Face

Master Cylinder Piston Rear "Brass" Washer

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

Master Cylinder Piston Rear Seal Orientation

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 Retainer Cup Installed

Master Cylinder Piston Rear Seal and Metal 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

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

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 Grommet on Front of Master Cylinder Piston

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

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 Reservior

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

Two Hole Master Cylinder Parts

Master Cylinder Spring Inserted in Bore

Master Cylinder Spring Inserted in Bore

Master Cylinder Inserted in Bore

Master Cylinder Piston Assembly Inserted into the  Bore

Pin Inserted In Hole in Rear of Master Cylinder Piston Assembly

Brass Colored Pin Inserted Into the Hole in the Rear of the Master Cylinder Piston Assembly

Master Cylinder Hinged Lever Arm Engaging Pin

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

Brake Light Switch & Metal Gasket

Master Cylinder Assembled

Master Cylinder Assembly from Rear

Front of Master Cylinder with Front Brake Light Switch Installed

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

Master Cylinder Installed on Frame Tube

Front Brake Light Switch Installed in Front of Master Cylinder

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_CaliperDetail_Source-MAX BMW

R75/6 Caliper Parts Detail-Source: MAX BMW

R75/6 Caliper Seal Kit - 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 With "40" Casting Mark

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

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

Fixed Pad with Wire Retaining Clip

Movable Pad with Flat Face Forward Slides Out From Caliper Piston

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.

Caliper Showing Piston and Dust Seal

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

Rag to Catch Piston

A Blast of Compressed Air Removes 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

Piston Free of Caliper But Dust Seal Still Stuck to Caliper Body

Old Dust Seal Shredded When Removed

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 Cover Metal Ring Left in Caliper Bore

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

Use A Pick To Push Dust Seal Metal Ring Out of Caliper Bore

Dust Seal Metal Ring That Separated From the Dust Seal

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

Use A Pick To Remove The Caliper Piston Seal Ring

Piston Sealing Gasket Removed

Piston Sealing Gasket Removed

The piston is hollow in the back where the brake fluid presses on the piston.

Brake Fluid Side of 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

How Dust Seal Fits Into Large Groove Near Front Face of Caliper Piston

Piston with Dust Seal

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

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

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 Soaked in DOT-3 Brake Fluid

Dust Seal Installed in Large Groove in Piston

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

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

Brake Caliper Eccentric Pin & New O-ring

Pulling O-ring On Eccentric Pin With Pick

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

Fixed Pad with Raised Locating Bosses

Movable Pad With Only The Center Pin

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

Fixed Pad “Hair Pin” Clip & Movable Pad O-ring

Movable Pad with O-ring Installed on Center Pin

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

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

Caliper & Pads Installed

Fixed Pad Secure with Wire Clip

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

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

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

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

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

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

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.