VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 Brake fluid coming out master cylinder cap

I went to bleed the brakes on the new system I installed and when you press down on the pedal it shoots brake fluid out the vent hole in the cap of the master cylinder. The system will pump fluid and I'm not getting air out of the bleeders put the pedal goes to the floor and it won't build pressure. I've never seen it come out the cap, what could be causing this?

MM,

The only thing I can think of is that maybe, just maybe, the front one way valve in the front is in basackards. But, I haven't thought this through enough to be positive about this. Did you rebuild the unit yourself? If so, I would check that all the internals in the master cylinder are in proper order and turned the right way. Mercy!

I'm still thinking. It's gotta be something simpler than what I suggested.

Don't tear it apart yet. Maybe someone else has experienced the same thing, but I never have. Let's hope you don't have to tear the MS apart.

As soon as Gene, Chipper, Tiny, Eastern Ed, MT, JD and a few others wake up this morning (They need their beauty rest), I bet you'll get a lot better answer than mine. I hope so.

Good luck,
Charlie

It's a new master cylinder that I purchased, so it wasn't something that has been rebuilt.

If there is a blockage in the outlet of the MC or brake line(s) so the air in the system gets compressed quickly it will cause the fluid to squirt into the reservoir and possibly out the vent. Many MCs will have fluid pushed into the reservoir at the beginning of the stroke particularly if the plunger is not properly positioned (plunger not far enough into the bore of the MC).

I would suggest bench bleeding the MC to see what is happening. If you install a tube into the outlet ending in the reservoir and push the plunger fluid should be pumped out the outlet and into the reservoir. If anything else happens then get a new MC or have that one taken apart to see what was done wrong or not working.

I did bench bleed first, so what you are suggesting since its mounted under the car now is to pull the rear outlet and fashion something to run from that back into the reservoir with the cap off?

If you bench bled the MC and did not note a problem then more than likely the problem lies in the brake lines. Maybe someone else has experienced the same problem.

Check the toe board adjustment.

I'll go back and check that. Would that affect the position of the rod position? It seems like its allowing the fluid back into the reservoir before its getting out the back.

If the piston is not far enough into the bore it will squirt brake fluid back into the reservoir.

I adjusted the toe board clearance to as far out as I could go with the cam, 7/16" and yet at the beginning of the stroke I still blow fluid out the vent cap. If I slowly start to stroke down about an inch and then start to pump them without letting the pedal come all the way back up then it starts to build some pressure(not great though) and doesn't blow out the cap. Would disconnectng the line at the first junction and just letting the fluid go out the back tell me anything? I know someone mentioned blockage in the lines, but they are all new, even the rubber lines. I'll try bleeding them again when my wife can assist tomorrow but I didn't know if there was anything to check first.

I have a '40 and twice during the 48 years I've owned and driven the car some 350,000 miles I've had brake fluid squirt out the MC cap vent hole. Both times the squirting was caused by the MC rubber cup being worn. Both times I could still get brakes but they would not hold and I had to continuously pump the brakes to stop. Once I replaced the cup the squirting stopped. Apparently when I pumped the brakes enough fluid was leaking past the cup to pressurize the reservoir and squirt fluid out thru the vent hole. I'm kind of a shade tree-er and have rebuilt my master cylinder in situ by removing the rear end cap and installing new parts without pulling the master cylinder. Since you say your MC is new I suspect the cup was either installed incorrectly or is just bad. The only time I go to the trouble of pulling the MC is if it is pitted. I've only had to do that once in 350,000 miles. I really hate messing with the clutch and brake pedals and even tho you mark their locations for reinstallation they are still troublesome at times.

As a followup I obtained another master cylinder and that was the issue because the pedal came right up and is firm on this one without it squirting out the cap.