About a year and a half ago I replaced my original brake master cylinder which I thought was leaking from the back where it connected to the booster, now Im not too sure.  I replaced it with a new GM master cylinder. To top off the new master I installed a chrome lid, new gasket and matching bails. I had trouble with fluid leaking from the back edge of the master from the lid, I removed the chrome lid, bails and gasket and replaced them with the GM ones that came with the new master..... same problem.  I can use a mirror to trace the fluid from the rearmost edge of the master then it runs down the back of the MC to the bottom  where it drips onto everything below, bubbling and removing the paint from my chassis.  Has anyone else ever had such a problem?  What could be causing this? The mating surfaces of the master and the gasket look perfect. Is there some weird circumstance that could pressurize the master causing it to do this?  Brakes work great and I dont want to replace the master unless I have to, just want the drips to stop.
Crossfire19822009-12-28 13:45:36

I have to wonder if the rear bail puts just a bit too much pressure on the lid causing it to lose its shape. As if the mounting holes for the rear bail are slightly incorrect.

Try lowering the fluid level in it.
Also, you say the surface of the Master looks "perfect"...do you mean it is machined flat?
The bails may not be holding the top on tight, or the lid may be distorting when the bails are pulled on. Leaks like this are fairly common, but they really shouldn't be happening anyway.
Have you looked closely for a possible crack in the master?
Adams' Apple2009-12-29 06:11:06

Thanks Guys.  Ill check the master closely for cracks when I finish with the project Im doing now (park brakes).  Joel, what I meant was that the machined surface on the top of the master looked flat with no deformities.  Kevin, I hadnt thought of the bails being too tight causing the gasket to deform, thats a possibility. Crossfire19822009-12-29 06:02:08