Replacing break master cylinder kit on '81 R65

[R65Biker]

Hi Guys,
Can anybody give me a few hints on replacing the break master cylinder (seal kit) on this bike? I have a small leak and am sure the agents will charge an arm and a leg to do this.
My plan is to use the attached drawing as a guide. Do you recommend I drain all the brake fluid from the system while I'm at it?
Thanks

[Bob_Roller]

You have to match the rebuild kit to the diameter of the master cylinder .

There is a number cast into the master cylinder, you can see it if you look at the bottom side of the part .

Should be 12, 15, 16 , corresponds to the diameter of the master cylinder bore  in mm .

Just make sure you get the correct kit .

Master cylinder is not a difficult part to rebuild .

Only difficulty you probably will have, is getting it removed from handle bar fitting, usually corrosion starts between the two parts and makes it a job to get the two separated .

Drain and flush the old fluid out .

When you get time, it would be a good idea to remove the caliper and dis-assemble it, clean it out and replace seals there as well .

[Ed Miller]

Earnst, when I got my R65 and had to go through the front brake system, BMW sold me an entire master cylinder piston assembly.  It was all put together.  All I had to do was remove my old one (corroded, rusty, etc.), clean out the bore really well, and put in the new one.  It was about $110 or so.

...yeah, drain out all of the old fluid.  

[R65Biker]

Thanks guys,
I could not wait for the weekend to do this. I want the bike in top condition and ready for a trip at any time.
Bought the kit and replaced it. I battled to flush the system because there was no flow through the pipes. What happened was - small pieces of the almost completely disintegrated seal was pushed down the left and right hand side hoses. Or so it seemed - tiny black goo that looks like rubber. One side little flow the other completely blocked. Used a thin wire and comressed air to get them open.
Rest was a piece of cake. Now stopping with a much improved force and the leak is also fixed. Should now fly through road worthy when I take it to get registered on my name. Thank you very much.

[Justin B.]

I had to roto-root both brake lines on the Dawg when I put it back together...

[Matt Chapter]

I just installed a stainless steel line for a similar reason, it was very difficult to bleed the brake system, even after a master cylinder and caliper rebuild.  At first I suspected it was the brake line collapsing but I suppose it could also have been a clogged line.  I'm quite happy to have spent the 80ish dollars to get the stainless steel line versus trying to clean out the old rubber line though.

[nhmaf]

If the brake lines were that full of gunk, I'd recommend replacing them.  If you can get some new stainless steel braided lines, you'll be impressed with the increase in braking power.

[Julio A.]

I recently replaced my master cylinder piston, when I bough mine from BMW it did came with all the rubber and spring installed. All I got to do was to swap it out with the old one. Fixed the brake fluid leak there. I'm happy I did open it up, the thing was full of gunk and god knows what inside.  

You should drain the fluid if time allows it. Prevents the paint destroying stuff from going all over the place and removes any build up of dirt and stuff that accumulated over the years. Nothing bad about having a fresh set of brake fluid. Although bleeding it is a pain for one person to do though.

If anything is stuck, like the reservoir to the handlebar fitting due to old brake fluid build up or corrosion, slight taps with a rubber mallet will help.