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:

• 34 BMW 1983 R100RS Remove Brake System

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:

• 34 BMW 1983 R100RS Disassemble, Inspect & Rebuild Brembo Brake Calipers

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.

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. 🙂

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.

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 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.

Inspection – FAIL 🙁

Inside is the piston, seals, and return spring.

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. 🙁

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.

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.

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.

The rear master cylinder is made by Brembo and mine was 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.

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

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

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.

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.

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.\

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.

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.

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.

NOTE:
In the above picture, the dust seal parts on the left end of the piston rod are not shown in the proper order. I dropped the piston assembly when I removed it and didn’t put the seal parts on the rod in the correct order. I show the correct order later in the assembly pictures.

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. 🙂

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, one-time crush ring and flat washer.

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

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).

One side of the one-time crush ring has a flared edge that is rounded. The other side has a straight edge

The flared end of the one-time crush ring slides over the flat face side of the seal so the flare butts against the lip of the seal.

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

Then the seal with one-time crush ring goes on with the “Brembo” face of the seal up against the flat washer and the straight edge of the one-time crush ring on the flat face side of the seal.

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

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

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

Install Piston and Assemble Rear Master Cylinder

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

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

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

Revisions

2020-06-13  Clarified parts order and terminology of the dust seal and one-time crush ring.