34 BMW 1983 R100RS Disassemble, Inspect & Rebuild Master Cylinders

This bike has two master cylinders, one for the front brakes that is integrated into the right throttle assembly and the other for the rear brake mounted on the inside of the right side rear foot peg bracket above the swing arm. I show how to remove them here:

The brakes on this bike are made by Brembo and so is the rear master cylinder. However, the integrated front master cylinder is made by Magura who supplies the throttle assembly. There are master cylinder rebuild kits available for both the front and rear master cylinders.

I also rebuilt the three brake calipers and you can read about how I did that work here:

Parts

Each master cylinder has a different rebuild kit. Since my front master cylinder was rusted, I only purchased and installed the rear rebuild kit, but I list both master cylinder rebuild kits in the table below. Both kits are available from Euro Motoelectrics, where I got my rear master cylinder rebuild kit, as well as from your local BMW dealer.

To further complicate things, I managed to damage the throttle assembly denting the thin-walled tube the throttle slides inside of when I dropped it on the floor and inadvertently stepped on it. 🙁 My attempt to repair the dented housing broke the thin wall.

Broken Throttle Assembly Casting :-(

Broken Throttle Assembly Casting 🙁

I decided to replace the throttle assembly, fluid reservoir and front master cylinder as this entire assembly costs just a bit more than buying the throttle assembly casting and new master cylinder assembly. So, the front master cylinder and throttle assembly should be good for another 36 years or so. 🙂

New Throttle Assembly With Front Master Cylinder

New Throttle Assembly With Front Master Cylinder

Part #                    Description                                         Qty
REAR MASTER CYLINDER
34 21 1 242 791 SET: REPAIR KIT BRAKE MASTER CYLINDER (from 09/80), Rear 1
34 31 1 238 040 PROTECTION CAP, Rear MC 1
35 21 1 238 039 PLATE, Rear MC Linkage 1
07 11 9 987 821 FORKHEAD – G6X12-CD, Rear MC Linkeage 1
35 21 1 238 038 TENSION SPRING, Rear Brake Pedal 1
16 12 1 116 458 HOSE CLAMP, Rear MC Fluid Line 1
34 32 1 236 259 HOSE, Rear MC to Reservoir 1
35 21 1 236 231 PULL ROD, Rear MC Linkage 1
07 12 9 934 961 PIN – 6X12, Rear Brake Pedal Pin to MC 2
FRONT MASTER CYLINDER
32 72 2 302 372 BRAKE MASTER CYLINDER – D=14,29, Front MC Rebuild Kit * 1
32 72 2 302 368 HANDLE UNIT, RIGHT – D=15MM (from 09/81), Throttle Assembly w/ MC 1
34 32 1 242 205 HOLLOW BOLT (from 09/80), Front 1
07 11 9 963 072 GASKET RING – A10X13,5-CU (from 09/80), Front 2

* NOTE
I didn’t buy the front MC rebuild kit as I replaced the entire throttle assembly (part# 32 72 2 302 368) which includes a complete front master cylinder assembly.

Disassemble Front Master Cylinder & Inspect

The front master cylinder has a 15 mm bore, as indicated on the body, and appears to be manufactured by Magura who supplies the left side throttle assembly.

Magura Master Cylinder-15 mm Bore

Magura Master Cylinder-15 mm Bore

There is a front dust seal and large flat washer that are installed in the throttle assembly. The seal fits over the end of the piston nose and the flat washer protects the seal. The brake lever presses on the end of the piston nose to pressurize the front brakes.

Front Master Cylinder Piston Nose Extends Through Throttle Housing to Engage With Brake Lever

Front Master Cylinder Piston Nose Extends Through Throttle Housing Against The Brake Lever

Flat Washer Protects Front Seal

Flat Washer Protects Front Seal

Front Seal Fits Inside Throttle Housing

Front Seal Fits Inside Throttle Housing

Front Master Cylinder Video

I made a short video showing how I disassemble it.

Remove Piston Assembly

The front master cylinder piston assembly is secured inside the cylinder with a C-clip. I use my C-clip pliers to remove it.

C-Cip Retains Front Master Cylinder Piston Assembly

C-Cip Retains Front Master Cylinder Piston Assembly

C-Clip Pliers

C-Clip Pliers

Use C-Clip Pliers To Remove C-Clip From Front Master Cylinder

Use C-Clip Pliers To Remove C-Clip From Front Master Cylinder

Inspection – FAIL 🙁

Inside is the piston, seals, and return spring.

Front Master Cylinder Disassembled

Front Master Cylinder Disassembled

Unfortunately, all of these parts are rusty. That means the bore of the master cylinder is likely pitted. Unfortunately, particularly with old bikes like this one, folks park them and don’t change the brake fluid for years. But, brake fluid is hydroscopic meaning it absorbs water from the air all the time. Eventually the water settles out and corrodes the steel master cylinder and internal parts. 🙁

Piston And Spring Removed Are Gunky and Very Rusty :-(

Piston And Spring Removed Are Gunky and Very Rusty 🙁

I use an LED flash light to look inside the master cylinder and it’s not pretty even after I cleaned it out with brake cleaner. So, I will have to replace it rather than rebuild it.

Front Master Cylinder Bore Looks Bad

Front Master Cylinder Bore Looks Bad

Front Master Cylinder Bore is Pitted - I Will Have To Replace It :-(

Front Master Cylinder Bore is Pitted – I Will Have To Replace It 🙁

How To Install Front Master Cylinder Piston Assembly

Since I managed to damage the left throttle control housing and bent it I dropped it and the managed to step on it, :-(, I’m going to buy the left side complete throttle control assembly. Consequently, I won’t be able to show how I install the Magura front master cylinder rebuild kit on this project, but I can describe how this is done.

The front master cylinder piston assembly comes in one piece with a front seal and large flat washer that are installed inside the throttle assembly where the front brake lever mounts. The seal fits into a hole in the throttle housing and the master cylinder piston nose fits inside the seal. The flat washer fits in front of the seal to protect it as shown in the beginning of the “Disassemble Front Master Cylinder & Inspect” section above.

New Front Master Cylinder Piston Assembly [SOURCE: Euro Motoelectrics]

New Front Master Cylinder Piston Assembly [SOURCE: Euro Motoelectrics]

If I was to install it, I would soak the piston assembly in some DOT 4 brake fluid for several minutes. I would liberally use the brake grease on the seals and put some inside the bore of the master cylinder to make it easier for the piston assembly to slide into the bore.

Then I would put the assembly inside the bore of the master cylinder starting from the return spring end of the piston assembly.

I would secure the piston assembly inside the master cylinder with a new C-clip.

Finally, I would install the front seal into the throttle assembly, attach the master cylinder to the throttle assembly, add the flat washer and then install the brake lever.

Disassemble Rear Master Cylinder & Inspect

The rear master cylinder is attached to a bracket that mounts on the inside of the right rear passenger foot peg bracket. The rear brake foot pedal connects to an actuating lever that pushes  on the end of the master cylinder piston. The rear brake fluid reservoir and hose attach to a nipple on the body of the master cylinder. I removed the master cylinder from its mounting bracket and the rear brake pedal when I removed it from the bike.

Rear Master Cylinder and Brake Pedal Assembly

Rear Master Cylinder and Brake Pedal Assembly

Rear Brake Master Cylinder Removed From Bracket and Rear Brake Pedal

Rear Brake Master Cylinder Removed From Bracket and Rear Brake Pedal

The rear master cylinder is made by Brembo and mine was manufactured in February 1981.

Rear Master Cylinder Manufactured by Brembo

Rear Master Cylinder Manufactured by Brembo

Rear Master Cylinder Brembo Casting Number

Rear Master Cylinder Brembo Casting Number

Casting Mark On Rear Master Cylinder Indicates Manufactured in February 1981

Casting Mark On Rear Master Cylinder Indicates Manufactured in February 1981

Rear Master Cylinder Video

I made a short video on disassembling the rear master cylinder and rebuilding it.

Remove Brake Fluid Reservoir Hose

The brake hose is secured to a nipple on the master cylinder with a use once crimped connector. I use my Dremel tool with a cut off wheel to cut through the crimp on the steel band.

WARNING
I don’t want to cut through the band where it wraps around the hose as I risk damaging the nipple inside the hose should I get too aggressive with the cut off wheel. So I cut the crimped end off the band.

Dremel Tool With Cut Off Wheel

Dremel Tool With Cut Off Wheel

Cutting Clamp on Rear Master Cylinder Fluid Reservoir Line

Cutting Clamp on Rear Master Cylinder Fluid Reservoir Line

Rear Master Cylinder Brake Fluid Line Clamp Ready to Remove

Rear Master Cylinder Brake Fluid Line Clamp Ready to Remove

I use a screw drive to spread the ends of the clamp and pull the hose off the nipple.

Rear Master Cylinder Brake Fluid Line Nipple

Rear Master Cylinder Brake Fluid Line Nipple

There is a part number and the bore size of the master cylinder stamped into the master cylinder next to the nipple.

Rear Master Cylinder Markings -14 mm

Rear Master Cylinder Markings -14 mm

Remove Rear Brake Pedal Actuating Lever Assembly

Next, I remove the brake pedal actuating lever assembly. It mounts on a pin that goes through two bosses on the master cylinder and is secured on each end with an E-clip that fits into a groove of the pin. The pin also secures the rubber boot that keeps dirt out of the end of the master cylinder.

Rear Master Cylinder Assembly & Bracket Detail

Rear Master Cylinder Assembly & Bracket Detail

C-clips Secure Pin Holding Brake Pedal Actuating Lever Assembly to Rear Master Cylinder

E-clips Secure Pin Holding Brake Pedal Actuating Lever Assembly to Rear Master Cylinder

I use two screw drivers to push the E-clip off the ends of the pin and remove them and the flat washer underneath them.

Use Screw Drivers To Remove Actuating Lever Linkage C-clips

Use Screw Drivers To Remove Actuating Lever Linkage E-clip

Actuating Lever Assembly C-clip

Actuating Lever Assembly E-clip

Rear Brake Pedal Actuating Lever Assembly Pin C-clips & Washers

Rear Brake Pedal Actuating Lever Assembly Pin E-clips & Washers

I mount the master cylinder in my vice using the soft jaws. I use a drift to drive the pin out and remove the brake pedal actuating mechanism from the master cylinder.\

Use Drift To Drive Out Pin Of Actuating Lever Assembly

Use Drift To Drive Out Pin Of Actuating Lever Assembly

Rear Brake Master Cylinder Foot Pedal Linkage Removed

Rear Brake Master Cylinder Foot Pedal Linkage Removed

Remove Piston Assembly

Unlike the design of the front master cylinder, the rear master cylinder piston is not retained in the master cylinder bore with a C-clip. The actuating lever assembly keeps the piston captive in the bore. I have to drive the piston out of the bore from the threaded hole the brake line screws into.

Brake Line End of Master Cylinder Showing Small Hole

Brake Line End of Master Cylinder Showing Small Hole

One suggested method is to use compressed air to push it out. You have to put your finger over the brake fluid nipple while you do this. I mount the rear master cylinder in the soft jaws of my vice and put some shop rags underneath where the piston will come out so it doesn’t smack into the vice or the floor and apply air pressure. In my case, this did not work.

Master Cylinder Mounted in Soft Jaws of Bench Vice

Master Cylinder Mounted in Soft Jaws of Bench Vice

Using Compressed Air To Remove Piston Assembly-Fail

Using Compressed Air To Remove Piston Assembly-Fail

The other method is to use a small drill bit, 3/32 inch works, and push it through the hole at the back of the threaded hole and tap it out with a hammer. This worked for me.

3/32 Drill Bit Used To Drive Piston Assembly Out of Rear Master Cylinder

3/32 Drill Bit Used To Drive Piston Assembly Out of Rear Master Cylinder

Insert Drill Bit Through Orifice in Brake Line End of Rear Master Cylinder

Insert Drill Bit Through Orifice in Brake Line End of Rear Master Cylinder

Knocking Piston Out of Rear Master Cylinder Using Small Drill Bit As A Drift

Knocking Piston Out of Rear Master Cylinder Using Small Drill Bit As A Drift

Piston Assembly Removed From Rear Master Cylinder-No Rust

Piston Assembly Removed From Rear Master Cylinder-No Rust 🙂

Inspection – PASS 🙂

The piston assembly is not rusted so that’s a good sign. All the piston assembly parts are replaced in the rebuild kit. I clean out the bore of the rear master cylinder with brake cleaner and inspect it. There are no signs of rust or pits so I can reuse it.

Polish Rear Master Cylinder Bore

I like to use 600 wet/dry paper to polish the master cylinder bore before I install the rebuild kit. A 1/2 inch drill bit fits inside. I wrap the 600 wet/dry paper around it one layer thick and secure with masking tape. I make sure the end of the bit is about 1/2 inch from the end of the sand paper and fold the paper over the bit to keep the end from scratching the bottom of the master cylinder bore. I run the paper in and out at a slow to moderate RPM while pushing it in and out of the bore. Then I clean it out with brake cleaner and use a strong flash light to inspect the bore. It shines like a mirror which is what I like to see. 🙂

600 Wet/Dry Paper On 1/2 Inch Bit For Polishing Inside of Master Cylinder

600 Wet/Dry Paper On 1/2 Inch Bit For Polishing Inside of Master Cylinder

After Honing-A Mirror Like Shine

After Honing-A Mirror Like Shine

Assemble Rear Master Cylinder Rebuild Kit

Before installing the master cylinder rebuild kit, I painted the master cylinder with brake caliper paint which is resistant to brake fluid. I protected the inside of the master cylinder and the nipple from the paint.

The kit includes a new piston with two large o-rings installed, a packet of grease for the rubber seals, and the front seal assembly that includes a rubber seal, stiffening ring and flat washer.

Rear Master Cylinder Rebuild Parts From Euro Motoelectrics

Rear Master Cylinder Rebuild Parts From Euro Motoelectrics

The piston has a larger diameter nose on one end with a slight step next to the face of the piston.

Rear Master Cylinder Piston With Factory Installed Seals

Rear Master Cylinder Piston With Factory Installed Seals

The front seal has a side with lettering including “Brembo” (hereinafter referred to as the “Brembo” face) and a part number. It also has a chamfer around the hole. The other side has no lettering and has two stepped flat faces (hereinafter called the “Flat” face).

"Brembo" Face of Front Seal Has Dished Hole

“Brembo” Face of Front Seal Has Dished Hole

"Flat" Face Of Seal Has a Lip

“Flat” Face Of Seal Has a Lip

One side of the stiffening ring has a flared edge that is rounded. The other side has a straight edge

Flared End of Stiffening Ring

Flared End of Stiffening Ring

Flared End of Stiffening Ring

Flared End of Stiffening Ring

Straight Edge of Stiffening Ring (To the Right)

Straight Edge of Stiffening Ring (To the Right)

Straight Edge of Stiffening Ring-To Left

Straight Edge of Stiffening Ring-To Left

The flared end of the stiffening ring slides over the flat face side of the seal so the flare butts against the lip of the seal.

Stiffening Ring Orientation On Flat Face of Seal (Straight Edge Faces Right)

Stiffening Ring Orientation On Flat Face of Seal (Straight Edge Faces Right)

Stiffening Ring Orientation On Front Seal

Stiffening Ring Orientation On Front Seal

Stiffening Ring Orientation On Front Seal

Stiffening Ring Orientation On Front Seal

Stiffening Ring Orientation On Front Seal

Stiffening Ring Orientation On Front Seal

The flat washer goes on the larger diameter nose of the piston and fits against the face of the piston.

Front Seal Fits On Larger Diameter Nose Of Piston

Front Seal Fits On Larger Diameter Nose Of Piston

Front Seal Flat Washer

Front Seal Flat Washer

Flat Washer Goes On First Against Face of the Piston

Flat Washer Goes On First Against Face of The Piston

Then the seal with stiffing ring goes on with the “Brembo” face of the seal up against the flat washer and the straight edge of the stiffening ring on the flat face side of the seal.

Flat Face of Stiffening Ring and Flat Face of Seal Face You

Flat Face of Stiffening Ring and Flat Face of Seal Face You

The front seal assembly is in a sandwich with the flat washer on one side of the lip of the seal and the stiffening ring on the other side of lip.

Front Seal Assembly Parts Orientation

Front Seal Assembly Parts Orientation

The return spring fits onto the smaller diameter nose of the piston.

Spring Goes on Smaller Diameter End of Piston

Spring Goes on Smaller Diameter End of Piston

Here is the piston assembly with return spring ready to install inside the master cylinder bore.

Piston Assembly Ready To Install In Master Cylinder Bore

Piston Assembly Ready To Install In Master Cylinder Bore

Install Piston and Assemble Rear Master Cylinder

Here are the parts needed to install the piston into the master cylinder.

Master Cylinder, Actuating Lever, Rubber Boot, Retaining Pin, Flat Washers & E-clips

Master Cylinder, Actuating Lever, Rubber Boot, Retaining Pin, Flat Washers & E-clips

Before I put the piston assembly inside the master cylinder, I soak the piston assembly in brake fluid for several minutes.

Soaking Piston Seals in DOT 4 Brake Fluid for 10-15 Minutes Helps "Season" Them

Soaking Piston Seals in DOT 4 Brake Fluid for 10-15 Minutes Helps “Season” Them

Then I liberally apply the brake grease onto the two factory installed seals of the piston and on the front seal. I put more grease inside the master cylinder bore with my pinkie finger to help the piston assembly to slide inside the master cylinder.

Seal Grease Eases Insertion of Piston Into Bore of Master Cylinder

Seal Grease Eases Insertion of Piston Into Bore of Master Cylinder

I insert the spring then the small diameter nose end of the piston into the bore. I put the flat washer on the larger diameter nose, then the front seal with the stiffener ring so the Brembo side goes against the washer. Then I push the piston all the way into the master cylinder to seat it in the bore.

Install Return Spring First

Install Return Spring First

Grease Applied to Piston Seals And Ready To Install

Grease Applied to Piston Seals And Ready To Install

Install Small Diameter Nose End Of Piston Into Master Cylinder

Install Small Diameter Nose End Of Piston Into Master Cylinder

Install Flat Washer Against Face of Piston

Install Flat Washer Against Face of Piston

Install Front Seal Stiffening Rings So The Brembo Side Faces The Flat Washer

Install Front Seal Stiffening Rings So The Brembo Side Faces The Flat Washer

Push Piston All the Way Into Master Cylinder Bore

Push Piston All the Way Into Master Cylinder Bore

Before I install the actuating lever, I put a small dab of brake grease inside the bronze busing of the lever and on the faces to lubricate it. Then I lubricate the rubber boot with some Ruglyde to make it easier to stretch it over the lever arm and body of the master cylinder

Brake Grease For Lubricating Lever Arm Bushing

Brake Grease For Lubricating Lever Arm Bushing

Using Q-tip To Apply Brake Grease Inside Bushing And On Both Faces

Using Q-tip To Apply Brake Grease Inside Bushing And On Both Faces

Lubricate Rubber Boot With Ruglyde To Easy Installation

Lubricate Rubber Boot With Ruglyde To Easy Installation

I put the pin through the bushing while pressing the actuating lever onto the nose of the piston. It is a bit tricky to get the hole in the lever to line up with the holes in the bushing.

Insert Lever Arm In Bushing And Push In On Piston

Insert Lever Arm In Bushing And Push In On Piston

Insert Pin Into Bushing While Aligning Lever Arm In The Bushing

Insert Pin Into Bushing While Aligning Lever Arm In The Bushing

Retaining Pin Installed Through Lever Arm and Bushing

Retaining Pin Installed Through Lever Arm and Bushing

Lever Arm Installed On The End of The Piston

Lever Arm Installed On The End of The Piston

Then I install the rubber boot over the end of the master cylinder and the plunger of  the piston assembly. I found hooking the opening over the lever and using needle nose pliers to pull the boot over the end of the master cylinder works well.

Install Rubber Boot Over Lever Arm First

Install Rubber Boot Over Lever Arm First

Use Needle Nose Pliers To Install Cover Over Master Cylinder Body

Use Needle Nose Pliers To Pull Rubber Boot Over Master Cylinder Body

Rubber Boot Installed

Rubber Boot Installed

Then I install a new flat washer and the original E-clip on each end of the pin. I use needle nose pliers to seat the E-clip into the groove.

Flat Washer Installed

Flat Washer Installed

Use Needle Nose Pliers To Install E-clip

Use Needle Nose Pliers To Install E-clip

E-Clip Installed

E-Clip Installed

Rear Master Cylinder Looks New

Rear Master Cylinder Looks New

I test the actuating lever to ensure it’s operating smoothly. Then I install the new hose onto the nipple of the master cylinder securing it with a new hose clamp. I slide the other over the nipple in the bottom of the rear brake fluid reservoir.

Replace Rear Master Cylinder Foot Brake Pedal Linkage

The parts are very rusty so I replaced them with new parts.

(Top) Rusty Original Brake Pedal Linkage (Bottom) New Parts

(Top) Original Brake Pedal Linkage Is Very Rusty (Bottom) New Parts

The 6 mm nut threads onto the threaded end of the turnbuckle. The plate that secures the rear brake pedal return spring goes on next to the nut.

Plate Fits on Threaded Half of Turnbuckle

Plate Fits on Threaded Half of Turnbuckle

But before I install the plate, I insert the “D” end of the spring into the hole. Then I put the plate on the threaded half of the turnbuckle and screw on the other half of the turnbuckle until it’s snug against the plate.

"D" End Of Spring Installed In Hole In The Plate

“D” End Of Spring Installed In Hole In The Plate

Brake Pedal Linkage Assembled

Brake Pedal Actuating Arm Assembled

When I install the master cylinder on the master cylinder bracket and attach the rear brake to the master cylinder, I will adjust the length of the turnbuckle so the rear brake pedal starts actuating the rear disk brake when I push my foot down on the brake pedal.

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