under tank master cylinder repair

How to repair the master cylinder that is under the tank

by Duane Ausherman

This posting is from 2004 and is the result of an ongoing thread.  This is the best description that got posted.  I have edited lightly for clarity and added comments.

You have found a leak.  Are you sure it’s leaking at the reservoir/casting junction?  A more commonplace leakage is at the rear end of the master cylinder.  I’ve had a couple of problems there on my R75/6 and R100/7. To find out for sure, remove the rectangular rubber boot at the rear of the master cylinder.  If it’s wet with brake fluid, that’s it!

What to do?  Remove the master cylinder; not such a big job.  Take off the reservoir again.  Probably you ought to remove the brake switch at the front end of the master cylinder, but I don’t.  Now put the nose of the master cylinder on the bench and push in the rear of the master cylinder piston with a Phillips screwdriver.  Lower your jaw slightly and note the residual brake fluid you’ve just pushed out onto the benchtop.  Now with the piston pushed in a bit, somehow manage to remove the circlip that keeps the piston secured in place.  Now the piston can come out.  If it’s stuck, push and release it a few times with the Phillips screwdriver.  The internal spring will soon push the piston out.

Time to inspect.  Look at the bore in the master cylinder.  It’ll be all ugly and nasty looking, and you’ll have a hard time seeing anything.  You’re looking for a pit about a half-inch inside the bore.  You may want to wash the bore out with warm soapy water.  What you’re going to do now is hone the bore.  I use a very fine strip of waterproof sandpaper stuck in a slit I’ve sawn in a 3/8-inch dowel.  Use 400 grit or finer.  Chuck the dowel in a drill and judiciously move it in and out of the bore using soapy water.

Now dry the bore with a rag and look inside.  Any pits?  If not, count yourself lucky and congratulate yourself on changing the brake fluid regularly. 

Got pits?  All is not lost. Use that same dowel and sandpaper and hone them out with the electric drill or a drill press, which makes it easier.  Hone up and down the full length of the bore but focus most of your energy on the pit itself. You’ll make the bore a bit oblong, but so what?  The seal on the piston is resilient enough to take care of a little eccentricity.

The pit will go away sooner than you imagine.  Keep on using soapy water and inspect often so you don’t go too far.  When the bore is cleaned up, you’re almost ready to reassemble the master cylinder.

You might have a problem with the seals on the piston. The good news is that new kits are available.  Take the front seal off the aluminum piston, noting the order of the parts very carefully.   If you screw up, the manual has an illustration of the correct order.

Now for the rear seal.  That’s the one that was leaking.  Don’t try to take it off yet.  To do so, it has to be severely stretched, likely destroying the seal.  Take your fingernail and loosen the parts up from each other. Now inspect the seal very carefully under a bright light, preferably with a magnifying glass.  Does the seal look worn, like a flat worn at the very tip?  Does that flat area look different from the area next to it, like less shiny or a different texture?  If you can’t make up your mind, there’s probably not enough wear to show.

Decision time to replace the seal or not.  If it’s worn, you have to.  If not, maybe you don’t.  Try it.  You don’t have that much to lose.

The reason for the monumental decision is that replacing that seal is a real bitch!  It must be stretched over a big lump of metal on that piston. The first time I replaced the seal, I was sure I was going to ruin it.  I used a probe, like an ice pick, and levered it over the ridge, much like you might mount a tire.

The second time I had to do it, I made a tool.  I have a small metal lathe, so I made a tapered thimble that ramped to a larger diameter so I could slide the seal up the ramp and let it sort of snap into place in the groove.  

There’s got to be a more elegant way to install that seal, but I haven’t thought of it yet.  Perhaps someone has a tip to share.

In any event, get the seal on, lubricate the rubber parts with brake fluid, and put everything back together.  I betcha it will work just fine.

Now all this might sound like a lot of trouble when you could buy a new master cylinder.  But new master cylinder bodies are expensive, so I decided it was worth it.  Or, more likely, I wasn’t going to admit defeat no matter how much work it took or the tools it took to build.  And I sure showed the guys who designed that master cylinder.

Ken Clevenger

This 2005 thread is related, so I just added it to this article.

I have owned this bike for 5 years and have ridden it quite a bit.  I had it out in California this summer riding on US 1.  The front brakes started dragging.  Not severely, but hard enough that the occasional push-start became impossible. I bought a wrench and bled the brake calipers.  Although the lever was not being applied, the fluid shot out of the caliper, so I quickly closed it.  As I took off the cover to the master cylinder reservoir, the pressure was released.  I closed it and continued the journey. the same brake sticking happened in only a few miles, and brake applications later.  This time I lessened the screws on the top of the reservoir and left them loose.
What is causing this?

I have experienced this same problem.  The hole in the bottom of the reservoir that Bill is referring to is the passage in which brake fluid is returned.  If this hole is blocked, the fluid used to apply the brakes has no way of returning.  Now the brakes drag and heat the fluid for a while, and it gets really interesting.  Don’t ask me why I know.
What you need to do is clear out that hole so that things can go back to normal.  You need first to locate an object which will ream the hole out.  I used a very fine drill bit (.083 dia???), but I’ve heard you can also use a single strand of braided electrical wire for this job.  In other words, it’s a very small hole and easily plugged.
 The next big task is finding the hole.  You will need to extract all or most of the fluid out of the reservoir so that you see the bottom.  I think there is a depression there; inside it, you’ll find this tiny hole.
Once you’ve located it, you can ream it out.  Then fill up the reservoir, bleed out the remaining old fluid, replace it with fresh, and you’re on your way.

Good luck, and make this a yearly routine.

Updated 30 March 2023