Rear Brake Master Cylinder

[Red]

My '85 K100RS rear brake master cylinder has started to seep a bit of fluid. Noticed the edge of rubber cover getting wet. I'm afraid to pull back the protective rubber, and will most likely find allot more wet fluid....

Is it better to just replace this unit with a new one? Does one rebuild this unit? Can this unit be rebuilt? Anyone have instruction on how to rebuild it, parts, seals, information, etc.? Is this rear Master Cylinder unit the same for my K100RS and a later K1100 bike? My internet search, so far, has results of allot on the K1100 but not the 2V K100 bike ... my bike is non ABS.

Checked with local BMW dealer and over the counter price for the rear Master Cylinder unit for my bike is about $155.00/ea. dollars.

No, I have not picked up an aftermarket manual yet ... but that is another question ... which one is better to get?

Thanks.

[frankenduck]

A lot of times it's just crud in the master cylinder seals that make it leak and just taking it apart and cleaning everything will make it stop leaking - especially on bikes that have sat for a while.

So the first step is to take it apart and inspect it before spending any money.

Once you have it apart:

Clean up the wall of the cylinder bore with some 0000 steel wool and inspect it for pitting.  If it's pitted then you need to replace the whole thing.

If the bore is good the seals are getting old then you can get a rebuild kit.

Many times you don't even need to replace the seals.

There's two master cylinders for the rear - 12mm & 13mm. As far as I'm concerning they're interchangeable and ABS/non-ABS doesn't matter.

The rear masters are the same for the 1100s.

[Red]

Thank you frankenduck for your response. I will move forward with this and take this Master Cylinder apart and inspect, clean ... hopefully I can stop the leak with a kit and new seals. I love to re-utilize older parts vs getting new parts. Since this bike, is new to me, I can hopefully learn a bit more about my bike and its parts.

[frankenduck]

You can take the piston out by removing the 3mm Allen head set screw.  Keep your thumb on the piston so it doesn't shoot across the room.

A little warning: You'll notice that there's a little bolt with a lock nut on the brake lever that allows you to adjust the brake.  Advice: Don't mess with it.  If you make it too tight then hard rear braking can make the piston stick on the set screw and lock your rear brake so you're best off just leaving that adjustment as is.

[mystic red]

Red, you'll probably come out just fine cleaning up your M/C but if it doesn't work out rebuild kits are out there. In fact, I'm 99% sure I have one in the shop I could part with.

[Mickster]

Too Legit to quit.

[Red]

As stated above, in my neck of the woods, a rear brake master cylinder runs around $155.00/ea over the counter at my local BMW dealer. I just ran down to BMW to inquire and a rebuild kit (13mm) cost about $73.00/ea. It would be easy to just buy the part(s) and install and have done with it. ... but I like the K bike partly because I can work on it and in some cases, the challenges the K brings forth to keep it runnin. Plus, I am, of the old school, that $10.00 bucks is still allot of money and I would rather spend that money on a hamburger than a bike part , if I can. My objective is to get everything on this K bike running correctly and all it's parts working so that I have the confidence to ride and go anywhere I want to. All of you have been talking about things I have an interest in ... the advise provided in this forum has been easy to understand and "right on", which is much appreciated. My next step is to take apart the master cylinder rear brake unit and clean it and put it back together, and hopefully it will operate properly. If not, I am a patient man, and will move on to the next step to fix.  Thanks all for your advise, pointers and support. I would love to meet you all in person, at some time.

[jjconeill]

my rear brake locked up. so i took it off put new pads in and it's ready, except the master cylinder is shot, it's a 12mm, i would rather purchase a new item ready to install, but it's $246 and money is tight, can i put a 13mm in it's place for $145. would make my wife happy.

[Martin]

Cheap easy replacement guide from $10.00 US. http://www.motobrick.com/index.php/topic,9364.0.html
Regards Martin.

[jjconeill]

I found this part # 34312310867 on ebay for less, but not sure if it is good for my k75s rear disc. MaxBmw shows this #  34 31 1 458 142 for my 1987. anyone know if it would work? Thanks