The Stovebolt Site Forums The Shop Area Driveline Master brake cylinder plug leaks

Why is there a hole in the center of the plug that comes out the side of the plug ,,,,and the rubber ring does not stop fluid from squirting out when stepping on the brake peddle .. ?

It's a vent, which must be there if the brakes are going to work correctly. If fluid is squirting out of that hole, the reservoir is either overfilled or there's a failure of one of the MC piston seals.
Jerry

I stuck a tooth pick in the hole and it did not squirt fluid out ,,and the brakes worked . does that plug have a valve in it that I cannot see that lets air in as I pump the brakes and then lets air out too? Thinking that I need a new plug ?

It is a very simple hole. No moving parts.

I agree it looks like a solid metal plug with a hole in the underside of the threaded end that continues out of the side of the plug,, the plug has a rubber ring washer and when I screw it in tight ,,,and then pump the brake pedal by hand ,, it squirts brake fluid out through the side of the plug where the hole is .. But after sticking a tooth pick in the side hole ,,it does not squirt fluid out when I use my hand and pump the brake pedal ,, I just don't get it ,, yet ..

I just reread your reply and now see that the first time I filled the reservoir I may of filled it to full and therfore made the fluid squirt out the side of the plug.. and so by the fluid level going down some ,,I might not of had to put the toothpick in the hole to stop the fluid from coming out .. so now I am thinking that I should take the plug out and get that tooth pick out of the hole and replace the plug and it will work fine ,, am I getting this straight like what you are telling me?

There needs to be a little air above the fluid, like the free space in a zero pressure radiator tank. The first thing that happens as you push the pedal will be a little bit of a fluid surge back into the reservoir before the piston starts to pressurize the fluid in the lines. Overfill the reservoir, and the surge will push some fluid out of the vent. Try using a turkey baster or something similar to remove an ounce or two of fluid- - - -I'll bet the problem goes away.
Jerry

I took the tooth pick out *made surer the level of the fluid was down some and put the plug back on and it is all working again great.. thx again Jerry

Moderator: moved to appropriate forum for better responses.

thx Pak 1 ,,I see now where I should of read more on the drive line forum and I would of seen the part about brakes.. I now could use some ideas of what I can wrap or put on the threads to the master cylinder plug as it seems to be messed up threads a bit , when tighten it up .. I read that plumbers tape will disolve if I use that.

By plug, are you talking about the reservoir cap? Can you clean up the threads with a chaser? I never put anything on the threads but I'd look for a new gasket or make one.

This is the 1 inch metal plug that fits into the metal master cylinder of my 39 Chevy,, it has a rubber "O" ring that goes on first before tightening

OK. I would use Permatex thread sealer or Permatex Seal Lock.

at Auto Zone

at Advance Auto Parts