Rear Master Cylinder

[byker]

I noticed today that the rear master cylinder was leaking. After removing the rubber boot and applying pressure to the brake lever, it was obvious that the cylinder is leaking. I have looked at my Clymer's and it says that you cant replace the seals on the piston, but rather you have to replace the entire piston as a unit. Is this so? If not, how do you order just the seals and what is the level of difficutly to rebuild the cylinder. It does not seem to be that difficult. Your help in this matter is appreciated.

Avrom Brown
Toronto, Ontario

[deilenberger]

I noticed today that the rear master cylinder was leaking. After removing the rubber boot and applying pressure to the brake lever, it was obvious that the cylinder is leaking. I have looked at my Clymer's and it says that you cant replace the seals on the piston, but rather you have to replace the entire piston as a unit. Is this so? If not, how do you order just the seals and what is the level of difficutly to rebuild the cylinder. It does not seem to be that difficult. Your help in this matter is appreciated.

Avrom Brown
Toronto, Ontario

Avrom - since you asked how difficult this job is - I'd recommend that you not attempt rebuilding the MC.

I find brakes to be fairly important on my K75.. and a motorcycle probably isn't the best place to learn how not to rebuild a master cylinder. A master cylinder that was improperly rebuilt has the capability of causing the brake to lock on - and that makes riding much more exciting than I like.

There is a secondary reason I'd recommend against this - the cylinder is VERY likely to either continue leaking, or start leaking again after it is rebuilt. The usual reason the cylinders fail is moisture in the brake fluid (caused by not performing the bi-annual fluid change BMW recommends) damages the bore of the master cylinder, which in turn - damages the seals. This same sort of damage can be seen if the front boot isn't fully seated or has any cracks in it. Once this sort of damage happens - even honing the cylinder will not remove all of it, and it will eventually cause the new seals to fail.

So - long story short - a rebuild should only be done by someone experienced in this sort of thing, and the success of the rebuild long-term isn't all that likely. I am experienced in rebuilding this sort of thing (youthful history with Girling hydraulics) - and I no longer attempt it.

Best,

[Kiltmeister]

As a followup to the Master Cylinder question;
some thing happened the other day in my garage when I tried to put the bike on the centre stand, it toppled over on the high side. After I picked up the bike I noticed that just beyond my wrist rest there is now a gap of about 3/8 of an inch where I presume that when the bike fell over it hit on the brake lever which in turn shifted the Master cylinder and the throttle lever over by this amount.
What is necessary to shift this back into the proper position??

[byker]

Thanks Don,

I will contact my local dealer and order a new rear master cylinder. Best regards and a Happy New Year.

[byker]

Don,

My dealer has a new master cylinder in stock and has put it on hold for me. Before installation, do you have to prime the m/c as required when I replaced the front m/c. After reading Clymer's, it says to fill the resevoir and "bleed" the system with no further instructions.

[Kiltmeister]

The best thing to do is to go to the Internet BMW Riders webpage and go to their tech pages area and there should be some information there.
www.ibmwr.org
The Kiltmeister

[glinm]

If you bench bleed (prime) the master cylinder before installation you have less air to work through the system when bleeding after installation. Just makes the job easier.

[deilenberger]

Don,

My dealer has a new master cylinder in stock and has put it on hold for me. Before installation, do you have to prime the m/c as required when I replaced the front m/c. After reading Clymer's, it says to fill the resevoir and "bleed" the system with no further instructions.

It won't hurt to prime it first, but look at the layout of the components. Ideally you want the master cylinder lower than the bleed point (on the caliper), and the reservoir higher than either.

Luckily - that's just how BMW laid them out :-)

I'm personally a big fan of a pressurized brake bleeder (see the IBMWR K-tech pages for a link to how to make one) - especially since I have ABS on my bikes, but this isn't necessary when installing a new master cylinder (IMHO - it IS absolutely necessary when bleeding/flushing a used master cylinder).

Best,