The front brake m/c on my '92-ABS is leaky... holds pressure for a few days but then gets spongy and eventually is completely empty, with a dribble down the side of my radiator. And that's just with the bike parked!

Looks like it's coming from the plunger area (that gets pushed in when one squeezes the brake lever).

Questions are:

1) How in the heck do I separate the m/c from the throttle assembly.

2) How do I separate the brake lever, so I can more closely inspect the plunger/sleeve.

3) Is there any point? I've heard that once these go, it's just better to replace them. If that's the case, will any K100 m/c work or do I need to locate an ABS specific model?

Thanks!

is the leak from master cylinder or a hole in hose if you have access to download section there a oem, haynes and clymer manuals
when i did my cylinder up the man at brake shop to me he could put a stainless steel sleeve in if i needed it but i didnt
master cylnder has screws on inside holding it onto throttle assembly

Master cylinder is the same one for all. I have never bought a new master cyl for a BMW. I always get a sleeve put in and they will out last the original.

its extreemly common for the seal on the plunger / brake lever to get leaky from what i have seen ......i had similar issues about 6 months ago ...the refurbish kit from motobins ..(obviously others as well ) is highly recomended ....

the local bm shop said ...."i wish i could buy that external seal by itsself "

i would suggest doing a light hone of the internal bore when replacing the seals ...they often become pitted from water absorbtion and later corrosion near where the seals sit in the bores

is really easy to undo the brake bits ......just becarefull of the brake fluid as it drops- falls off the parts and onto the painted areas .

hope that helps some

A '92 K100 prolly has a 20mm master cylinder with four piston Brembos. Any earlier and it'll be a 13mm with two piston Brembos. Kits are readily available for both.

The piston diameter is cast onto the housing - you should be able to see it in front of where the throttle cable exits the assembly.
Paul.

Not unusual with a leak on the 20mm m/c, due to the outermost piston seal is rather shallow in the bore.

Just a very few mm of corrosion eating its way down the bore from the outer end, and then the fun starts .

About dismantling: pivot bolt is mounted from underneath, and the female threads is in the lever...not the m/c housing.
Then it`s small screw on the side of the cylinder which keeps the piston inside the bore.

Have a look in this thread.

Thanks guys, I totally forgot I've got a perfectly good clymer manual sitting here on the bookshelf!

Hopefully it just needs a good cleaning, but if not where do y'all in the U.S. prefer to source your parts from?

Not sure how to quickly identify which master cylinder you've got, besides the obvious pairing info above? If you've a non-adustable throw lever it's a 13mm with two pistons calipers. Adjustable levers were used with the 20mm version and four piston calipers, and as Inge suggests, these are more prone to leakage due to design shortcomings. They are, however, all the same on 16V K100, K1100, and R1100 series. Piston and seal kits are $25 to $45, depending on who's the proudest.

Beemer Boneyard, South Sound BMW, Bob's BMW, or any number of other local BMW dealers. Or try Moto-bins UK for often the best price, and certainly excellent service.

Guest in the House wrote:Not sure how to quickly identify which master cylinder you've got, besides the obvious pairing info above?

Since it`s a ABS model (and -92), I would believe it got a 20 mm m/c.

I'm pretty sure mine's a 20mm as well.

Poking around under the outer rubber boot and it's definitely covered with crystalized fluid. I'm looking for my metric allen set to get this lever off and we'll see just what's going on inside!

I'm so eager to get this bike up and running! My buddies are torturing me with pictures of their weekly mountain rides.

On a side note, anyone know how much a used, mostly complete, RS fairing would go for? The PO dropped it one too many times on the right side and killed part of the panel, so it's all been stripped off and I want to get rid of it!