I have a 57 Mercury. replaced the master cyl, all the wheel cyl. and rubber hoses. bled the system everything seemed fine. I drove the car around town and before I could get home the brakes where all but locked up. I broke the line loose at the master relieved the pressure and it happened again. any suggestions? All stock single port mc, manual brakes.
you need free play at the pushrod, so the piston can move all the way back to uncover the relief hole. Do you have free play? or is the pushrod tight when the pedal is released?
Make sure the push rod that pushes on the mast cyl piston has a little free play when the brakes are not applied. Greg
I agree with everyone about having free play in the pedal. That's the first thing to check. Does it have power brakes? I had a Lincoln that was really hard on brakes. I could only get 10k miles before it needed a reline. One day at a friend's house while sharing a few brews, I noticed the brake lights come on on the Lincoln. Turns out that the booster would slightly apply the brakes from time to time. Replaced it and never relined the brakes again for the last 7 or 8 years I had it.
As Jim and a couple of others said, if the push rod is pushing the piston in even slightly it covers the compensating port and the fluid cannot return to the reservoir. I've seen it a couple of times when replacing the master cylinder the hole for the push rod in the replacement master cylinder's piston is slightly shallower than the one in the old one was.
Thanks for the help! I swapped the the plunger from old to new and that was the problem. Bam within 10 minutes you guys answered my question.