Strange master cylinder leak

[denydog]

After re-assemling my front master brake cylinder, I have a very slow but messy brake fluid drip from a intentional(?) hole on the bottom of the casting. I'm guessing it might be a pressure relief hole of some sort. I'm looking for any information I can get so I can decide how to deal with it.

[AOD]

TTT

I have this same hole/leak on my rebuild '83 1100E master cylinder. Does anyone know what this machined hole is for? It's right next to the brake switch area on the bottom of the master cylinder.

~Adam

[denydog]

Hey, a deja vu thread! I haven't seen this one in a while.

I came to the conclusion that the hole is just a drain, in case the MC piston leaks. Mine was apparently leaking because of the worn piston assembly. I just took mine apart to clean it and re-assembled with the old parts. Later, after I replaced it with a new piston assembly, the leak went away. I don't know what to say about yours, except that your piston seals must be leaking for some reason.

You seem to be having some tough luck! Did you get the brakes pumped up?

I was thinking some more about your question about which direction that one cup seal faces. That first picture in your other thread http://www.thegsresources.com/_forum...d.php?t=106630 , I think it shows your old piston assembly after you took it out? It looks like it shows the flat face of that seal on the outside, like I thought it went. I think I can just barely see it.

[AOD]

Deny,

I think something is fudged up in my master cylinder because it's not building any pressure if it's all going out the vent hole on the bottom.

I'll double check to make sure I have the snap ring on the correct seat...that's all I can think of besides causing internal damage.

Time to buy a M/C off ebay as a backup.

~Adam

[denydog]

My leak out the hole was a very slow drip, 2 or 3 drops per day at most. It was definitely fluid that seeped out past the piston seals. You should not have significant fluid flow unless some thing is major wrong. The hole should be on the outside of the piston seal area, so fluid should not normally reach the hole unless it's leaking past the seals.

Maybe your thought is right, and you don't have the snap ring/piston in all the way. The ring does tend to want to catch and stop at the first groove for the boot seal. Maybe losing fluid out the drain hole is causing your pressure problem, if it's a lot of fluid leakage. These MC's definatelly can be a little tricky. But I still would guess that yours is fixable.

[Commodus]

After re-assemling my front master brake cylinder, I have a very slow but messy brake fluid drip from a intentional(?) hole on the bottom of the casting. I'm guessing it might be a pressure relief hole of some sort. I'm looking for any information I can get so I can decide how to deal with it.

This happens when you don't seat the piston/cup assy properly. There are two grooves, one (the first) is for the outer rubber boot. If you seat the circlip in that groove, the piston will not seal properly. You have to push it down further into the cylinder and get it in the second groove. It's hard, I bought some special super long pliers to do it from mikesxs.com. The pliers are a bit expensive considering how cheaply made they are, but mikesxs.com is such a pleasure to deal with I bought them from there anyways.

This exact same thing happened to me and it took a while to figure it out. I even made a post about it here.

[AOD]

Yes, the above is true. If you have the piston/cup assembled wrong it will leak out the hole in the bottom.

My error came in that I put the spring and rubber seal in backwards. I pushed the rubber piece down in the bore instead of putting the spring in first. Oops. :?

~Adam