front brakes issue

[johnscf]

I’ve had this problem once before with a 67 1600 now having it with a 68 2000. It has a new master cylinder but sometimes it does not release the pressure and after a short drive with occasional brake application the front brakes are fully engaged and the pedal is all the way out and can’t be pushed. If I loosen the bleed screw on the master cylinder for the front brakes the pressure is immediately relieved and the brakes release. When this happened previously I bled the system a couple times and drove the car more often and it eventually resolved itself. I’m not sure if the bleeding the system or more frequent driving fixed the problem. It seems like the piston in the master cylinder is not being pushed back out after it gets depressed but as soon as you relieve the pressure by opening the bleed screw the brakes release and it’s bake to a normal pedal. Also had this problem with a 73 toyota celica but it also eventually went away. I’m using DOT 4 or DOT 3 brake fluid if that is a factor. Thoughts?

[Habitat.pat]

The master cylinder is not fully retracting the piston. If the push rod is adjustable, shorten it a bit at a time until the problem goes away, then take it another turn. If it is not adjustable, you will need to shim the master cylinder out until the problem goes away. If there were shims between the firewall & the master cylinder, did you put them back in?

Peace, Pat

[theunz]

If the rod is not adjustable you may be able to swap it with the one from your old master cylinder providing they are different lengths.

[Habitat.pat]

Good point , Mike!

[redroadster]

You say the brakes are engaged and the pedal won't give any farther .but the calipers are holding pressure , until you vent the line .
Sounds more of a caliper problem , a weak brake hose, cording broke will do something similar , or a MC jamming,
no pulsing in the braking ? hasn't had dot5 in it ?

[johnscf]

The master cylinder is not fully retracting the piston. If the push rod is adjustable, shorten it a bit at a time until the problem goes away, then take it another turn. If it is not adjustable, you will need to shim the master cylinder out until the problem goes away. If there were shims between the firewall & the master cylinder, did you put them back in?

Peace, Pat

looks like the rod is not adjustable. I’ll try adding shims today

[Gregs672000]

I'm curious as to how/why shims would solve this problem.

[Habitat.pat]

Since the pedal is against the upper limit, the pushrod is a fixed length & the piston is against the pushrod, the position of the piston is not adjustable. So In order to get the piston in a position where it isn’t blocking the bleed port in the cylinder, the cylinder must be moved forward to open the port. From the factory there were shims installed but a lot of them were removed when the master cylinder was replaced with one that had an adjustable push rod,, in fact I’ve seen it on this site where folks say that if the pushrod is adjustable to just leave the shims out. My pushrod is adjustable so I didn’t put the shims back in but I still have them.

Peace, Pat