BMW R850R Front Brake Master Cylinder Rebuild

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corroded master cylinder This isn't a step by step master cylinder rebuild. You'll want a shop manual for that. These photos are hopefully intersting if you are doing a rebuild, or trying to figure out if it is time to rebuild.

This is the front brake master cylinder of the BMW R850R (probably identical to the R1100).

With the cover, boot, and brake lever removed, you can see corrosion around the master cylinder piston. This has been leaking for a long time, and is long over due for a rebuild. I should have peeked up under here long ago. There should not be brake fluid in the dust cover. This wasn't a "leak" per se, but things were long past needing a repair.

The piston won't come out until you remove the retaining screw on the side of the master cylinder.

Carefully cover everything (especially painted surfaces like the tank and engine cylinder) below and around the master cylinder. Brake fluid is very corrosive to paint, and I'm pretty sure it's a carcinogen.


corrosion around the master cylinder pistonIf your master cylinder looks like this, you probably need a rebuild. The piston still moved fairly smooth, so I had plenty of braking force. The problem that we found was that the brake calipers weren't fully releasing. See the clogged return port in the next photo.
clogged reservoirIn the left side (left is towards the rear of the bike) of the master cylinder reservoir, notice the crystals that have built up. These have completely clogged the fluid return port.

The fluid return port is a tiny hole in the middle of the larger, left (rear) depression. I cleaned mine out with a single strand guitar string. You can see the depression, but the return port is not visible. You can see it in the next photo.

Brake fluid not returning when the brake lever is released has been known to expand due to heat during driving, and lock the front brake. Anton Largiader (our excellent local BMW motorcycle mechanic) noticed that my brake lever had far too little travel. He showed me a wrecked GS that this happened to - clogged return port, riding built up heat and pressure, the and pressure eventually locked up the front brake. Luckily, I don't think the driver of the GS was badly injured, but the bike was pretty badly thrashed.

I should have discovered the problem earlier. When I replaced my front brake pads, fluid would not return to the master cylinder. The pistons moved fine, but pushing one in forced the other out. If I pushed both, they wouldn't retract. I thought that was odd, but chaulked it up to some odd German engineering. Wrong. As for the lever travel, I also figured that was just the nature of the bike.
cleaned reservoirThe cleaned reservoir. Now your can see the tiny fluid return port. It's a little tiny hole in the middle of the circular depression in the lower left of the reservoir. The return port is easier to see in the larger image (click any image for a larger view).

The larger hole to the right is where fluid enters the cylinder of the master cylinder.
Another photo of the cleaned reservoir.
brake lever boltThis is the lever bolt. It comes from the factory with blue thread locking compound.

Between just normal grease and corrosive brake fluid, gloves are good. These are neoprene which seem more resistant to oil, etc. than latex.
questionable bolt headThe head of my lever bolt was pretty messed up. I didn't do this. As far as I can tell, previous service on the bike was good, so I don't know how this got messed up.
master cylinder pistonThe old piston didn't look too bad (if you ignore the rusty brake fluid crystals), but the rebuild kit has a new one, so don't bother reusing it. The real problem is that the rubber seal is shot, and the reservoir was all crudded up.
BMW R850R master cylinder rebuild kitThe rebuild kit. I don't have the part numbers. I seem to recollect that this is the "rebuild kit". It's a piston, return spring, retaining screw, and washer. However, to do the job you need more parts. I order my parts from Morton's BMW, and they are good about setting me up with all necessary parts.
additional partsThe other goodies are a new lever bolt because mine was messed up, a bushing for the bolt, a new master cylinder dust boot, and copper seals. I seem to remember removing the upper end of the brake line.

Of course, the whole front brake system had to be flushed with new fluid and bled. It was long overdue for new fluid.
cleaned master cylinder boreThe cleaned bore. I wiped this out, then used very fine emory paper to smooth the bore. Ideally, you'd use a very fine brake cylinder hone. Or ideally, you'd rebuild the master cylinder before it was corroded. Cylinder wall corrosion was minimal, so I decided to keep the master cylinder. It's an expensive part. This needs to be spotless. Be especially careful to remove any grit from the hone/emory paper. I cleaned the cylinder with denatured alchol, and then with a clean rag and clean brake fluid.
cleaned mc boreA wider view of the cleaned bore. I had some stuff taped out of the way. I used shop rags to cover the bike. I really should have taped plastic on things. I got a couple drops of brake fluid on my oil cooler covers. They aren't painted, and seemed to suffer no damage.