This posting is from 2004 and the result of an ongoing thread. This is the best description that got posted. I have edited lightly for clarity. Photos may follow some day, but don’t hold your breath.
You have found a leak. Are you sure it’s leaking at the reservoir/casting junction? A more common place for leakage is at the rear end of the master cylinder. I’ve had a couple 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 bench top. Now with the piston pushed in a bit, somehow manage to remove the circlip that keeps the piston in. Now the piston can come out. If it’s stuck just push and release a few times with the Phillips screw driver. 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. What you’re looking for is a pit or pits 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 on a regular basis. [I don’t often enough even though I know better. I guess that’s how I got to be an expert!]
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 at 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 very carefully the order of the parts. 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 and that will likely destroy the seal. Take your fingernail and loosen the parts up from each other. Now inspect the seal very carefully under a bright light and preferably 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 literally has to 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. Perhaps Ed Korn has a tool for that job.
There’s got to be a more elegant way to install that seal, but I haven’t thought of it yet. Perhaps some of you might have 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 just buy an new master cylinder. But new master cylinder bodies are really 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 tools it took to build. And I sure showed them, the guys who designed that master cylinder.
Now that was a lot of talk for something that might not have been your problem in the first place.
This 2005 thread is related, so I just added it to this article.
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 for a while and heat the fluid and it gets really interesting. Don’t ask me why I know.
So, what you need to do is clear out that hole so that things can go back to normal. You need to first locate an object which will ream out that hole. I used a very fine drill bit (.083 dia???) but I’ve heard you can also use a single strand of a braided electrical wire for this job. In other words its a very small hole and easily plugged.
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 and 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 replacing it with fresh and you’re on your way.