Brake pedal will not come back up

[mntoy]

Still having problems with the brakes on my 1974 Chevy C20 2wd truck. I have already replaced the booster and MC from Azone, benched bleed and installed. Took forever to bleed brakes and finally finished then drove the truck for 3 or 4 days about 80 miles with no problems. Get into the truck today pull out of the driveway and at the first stop sign I step on the brakes the truck stops makes a horn/whistle type noise and the brake pedal is stuck to the floor and the truck will not move (brakes stuck on) and the brake light was not on. Had to turn the truck off and push the pedal a couple times to make the pedal come back up. Started truck and put it in reverse and drove back home. Yes it happened a couple more times on the reverse trip home. One other strange thing is the MC cover leaks over the back reservoir only. The trouble shooting instructions for the Azone brake parts say the pedal push rod is to long and to shim between the MC and booster to shorten. Installed a set of body shims and still have the problem. Not sure what to think, can anyone help? Mike

[bmartin9122]

I had a similar problem with 3 (!!!!!) different rebuilt master cylinders from O'Reillys. I got frustrated because they would all bleed pressure. In the end, I had to track down a local company who could get me a new MC. Bingo! Problem solved. I am convinced the quality of rebuilt master cylinders is not what it used to be. Import labor perhaps?

Good luck!

[magnethead]

Originally Posted by bmartin9122

I had a similar problem with 3 (!!!!!) different rebuilt master cylinders from O'Reillys. I got frustrated because they would all bleed pressure. In the end, I had to track down a local company who could get me a new MC. Bingo! Problem solved. I am convinced the quality of rebuilt master cylinders is not what it used to be. Import labor perhaps?

Good luck!

agreed

[bmartin9122]

I just remembered the proportioning/combination valve that is mounted to the frame under the radiator. It has a pressure bypass built into it. However, it rarely if ever fails as far as I know, so I would try a better (new!) MC first before I plinked down something like $85 for a dealer-only part.

[mntoy]

Thanks guys, the MC was my first thought also. Can you explain how the poportion valve pressure by-pass can make the brakes stick in a closed or on position? I have a second parts truck that was in running condition about a month ago that I can change the porption valve with think I should try a swap? or go with a new MC first?

[bmartin9122]

Originally Posted by mntoy

Thanks guys, the MC was my first thought also. Can you explain how the poportion valve pressure by-pass can make the brakes stick in a closed or on position? I have a second parts truck that was in running condition about a month ago that I can change the porption valve with think I should try a swap? or go with a new MC first?

Even though I understand the purpose of the combo valve, I don't understand the exact mechanics well enough to be able to point to it as the source of your problem. However, if you get a NEW (not rebuilt) MC and you still have a problem, then that is about the only thing left - since you said you already replaced the booster.

I guess it is possible (though improbable) your brake calipers/cylinders are sticking in the closed position despite not having hydraulic pressure holding them there. I would be especially wary of the drum brake mechanism. I could see that complicated mess getting seized up over time if the truck hasn't been driven for a long period. When I rebuilt my brakes, I got some high quality white lithium grease in an aerosol can. After replacing the wheel cylinders and pads, I coated the rebuilt innards with grease being careful not to get any on the pads themselves.

I still think a new, high-quality (read: works properly) MC will solve your problem.