Brake flluid flowing out the top of the master cylinder

I call myself having rebuilt my brakes, replacing the brake lines, cylinders, and shoes at every wheel. I did this because, while driving my '75 CJ5 one day, the pedal, which had been getting lower, went to the floor. I got out of the Jeep and the master cylinder was empty. Anyway, I'm outside trying to bleed the brakes, and every time my assistant steps on the brake pedal, brake fluid leaks out of the middle of the front and the middle of the rear of the master cylinder cover.

Can anyone tell me what's going on? Do I have a bad seal between the master cylinder and the cap and need to replace both?

I appreciate any help y'all can give.

 

A good master cylinder will get a quick vertical squirt out of the vent/compensating port. If your cap seal is bad it will leak out as you describe. Is one reservoir level getting higher as you operate the master cylinder? If so the mc is bypassing (leaking internally) and needs rebuilt or replaced.

 

Sound more like,you got air in master cylinder. You need to pump the air out of system.

 

Always looking to learn something new. How will air in the m/c cause a fluid leak out the cap?

 

Similar situation happened to me when a bled my brakes a few months back. I didn't have it overflow and go out from under the cap, but bubbles were clearly seen in the mc in the fluid. May have happened because I didn't bench bleed the mc first.

 

In your case the air didn't cause the fluid to leak out the cap so the question remains. How will air in the m/c cause a fluid leak out of the cap.

 

When you have air in lines or m/c. And when you remove pressure from the pedal the air pushed the fluid back, and it hits the top of the lid and run along top and under gasket. And out and run down the sides.

 

As I stated, I had a similar situation but mine didn't overflow. I didn't want to overfill, but I do recall seeing fluid on the bottom side of the gasket from the rise in fluid. Fwiw my mc is a new dorman.

 

This is what I experienced when I put a new master on this year. Bend the hold down strap down in the middle so it puts more pressure on the cover. I've got mine tight enough I use a screwdriver to release it. Funny how modern plastic covers with little snaps seal so much better. Mine doesn't leak anymore.

I also removed the residual valves, probably made the back spray worse.

 

Me neither. (the following is not directed at you Patrick, I know you know how this stuff works)

The drum brake retracting springs will pull the shoes back, hence the pistons in the m/c back in their bores, and cause the fluid to go back to the m/c reservoir, which it's designed to do. Similar things will happen when the pistons retract back in their bores in disc brake calipers. In either case the fluid will not "squirt" up. In 25 years of working on brake systems, prototyping brake systems, and teaching brake systems, I've never seen air in the system cause it to squirt out of the m/c. I am interested in the theory WHY, or HOW this could have happened.
Might want to go back and re-read my first reply folks as I suspect that's what was really going on. Besides, if the cap and seal or gasket is making a good seal, it can't leak from the cap anyway.