I got my front disc brake set up installed and working nicely, however the Master cylinder is leaking from around the top cover. This is a basic GM dual reservoir disc/drum MC with the two "wire bales" that snap in place to hold the cover on. I filled the MC up right to the top edge after I was all done bleeding the brakes. Is that TOO much brake fluid? What is the correct level of the brake fluid supposed to be for the MC to be considered full? Should it be right to the top edge? Down a 1/2" from the top, or is there a fill line that I just don't know about and can't see? I took the bales off today and put a little more kink in the top/middle part so it would apply more downward force on the cover/gasket. I also added a piece of heat shrink to that same spot to add even more pressure. But if I have too much fluid in there to begin with, its probably going to find its way out.

 

What year? Is the cylinder slanted or level? In a level cyl, the fluid should be maybe 1/8 to 1/4 or so down. Same with slanted. Could be a bad gasket, or corroded or dirty gasket surface. Could be a bent cap. This is not really a typical problem. I don't know what was there before. You could have the wrong MC pushrod, wrong hole on the pedal arm, or wrong length MC pushrod.

 

I'm guessing when you put the cover on the gasket pushed out the fluid because you have it so high in the bowl. If you think about it you only have to have the holes in the bottom of the MC covered to maintain braking. If bled properly the caliper piston is barely moving as the pads are right up against the rotors.
The volume of fluid in the MC is there as a safety feature should you spring a leak. It will hopefully allow you to get to your destination before running completely dry but with a dual bowl system you would always have some braking capacity at least.
Personally I would have it at least a 1/2 inch down from the top so you're not spilling it everywhere. Brake fluid will remove paint, be careful.

 

The specified level is ¼" below the rim at the highest level of the fluid, so if it's an angled M/C, ¼" below the rear edge of the reservoir.

It's in the Chassis Service Manual.

- Eric

 

The MC is pretty much perfectly level, so I will remove enough fluid to get it down to that 1/4" below the top. Thanks for the manual info MDchanic.

 

On my '71 Supreme, the MC is angled. Even with keeping the fluid a 1/4" below the edge, I still had leakage from the cap.
I found that the bales weren't applying enough pressure to keep a tight seal. In order to test my theory, I placed a zip tie under each place where the bale contacts the cap.
Seeing as that did the trick, I snipped the ties down to 3/4".
This is my second summer leak free.

 

Cool idea. Thanks.

- Eric