disc brake lock-up

[daves68]

Just completed a front disc conversion on my 68 with a Scarebird  conversion.  The master cyl and booster were collected over the past couple of years.  The brakes were a little soft for the first few stops, then the pedal got harder and harder until the fronts were almost totally locked and would not release.
Master cyl  is the disc type with big/small chambers. Booster is an unknown rebuilt one that was supposedly for disc( is there a difference between disc and drum?) Proportioning valve for the rear drums and they do not lock up or drag.
I'm guessing I have a wrong booster problem. I would appreciate ANY input from all of you. Hot Rod Power Tour is in a couple of weeks and I can't go!!!
Dave

minocqua@prodigy.net

[John_Kunkel]

The pushrod at the front of the booster is probably adjusted out too long; this prevents the rear piston in the MC  (front brakes) from fully returning. If the rear piston doesn't fully return it doesn't allow the fluid to return to the reservoir and locks the brakes.

Loosen the nuts holding the MC to the booster and pull the MC forward, if the brakes unlock the pushrod needs adjusting.

[daves68]

Thanks John. I will try that right now. At wits end with this- afraid to drive it!

[toocheaptosmoke]

Sounds like maybe there's a residual pressure valve still in your front brake circuit?  Wrong the master cylinder, or front brake line on wrong port of master cylinder?

[daves68]

Where would the residual pressure valve be located?     I bought a M/Cyl with large and small chambers, I was told it should be for disc brakes. Brake line location checks out OK.

[toocheaptosmoke]

They're under the brass seat in the MC if so equipped, but not all MC's came with them.  Some MC's don't have them at all, and they could be located in-line somewhere else in the system.

[daves68]

Thanks for that pic. Tomorrow I will have to look for it.