I have replaced the master cylinder, the power booster and the Front calipers seem ok, replaced w/in last 5k miles. The Brakes work great at first (no sticking or pulling) but after a while the front brakes don't disengage after a full stop the pedal becomes rock hard. The only way to release the font brakes is to loosen the brake line into the master cylinder and go on without problems for a while. any ideas out there?

 

The only time I've experienced something like this was a bad caliper. But it also had close to 75,000 miles on it. John

 

my first guess is that the power booster is not adjusted correctly. if it is adjusted to where the master cylinder does not release all the way it could cause this.

 

that or a bad rubber brake line was my first thought but if it releases when he cracks the line at the master cylinder i would lean tord the booster adjustment.

 

I am going with the hoses although I wonder why both would do it at the same time. I had one that acted like a check valve in that it would only let fluid go one way. It would lock the wheel cylinder up until you crack the bleeder valve. This is the kind of stuff that makes you crazy and where this board really helps.

 

The power booster is new (not rebuilt) and from YearOne.

 

i have had rubber hoses do the same thing but if it releases when he cracks the line at the master cylinder it shouldn't be the hoses.

 

even a new booster can need adjusted. the rod between the master cylinder and the booster is where you want to adjust. i just replaced one on my wife's 72 cutlass and got lucky. it did not need adjusted.

i guess it is possible that the rod that goes to the pedal needs adjusted. i am not an expert on the adjustment but know it can cause what you are having.

 

Is there a separate position on the pedal for power and manual brake mas cyl's? If so, make sure it is on the power one. The manual position will put the push rod in the wrong place and give too much leverage.

 

Was there ever an answer to this. I have EXACT same scenario! Have to crack line at master do release. Everything is brand new disk conversion

 

Are the hoses new. They can deteriorate from the inside and look perfect on the outside. Kind of like a blocked coronary artery. The brake fluid is under tremendous pressure when the brakes are applied and it can force it through a restricted hose. When you release the brake, there is no "pressure" to push the fluid back, so it stays locked up. I had one do that on a 69 Grand Prix. The hose looked perfect. I replaced it and cut the old one open and it was swollen shut on the inside.

 

They are (from a conversion kit) but definitely an interesting point. I'll check it out. I was thinking vacuum line is to long and maybe collapsing. Was Gina try taking vacuum line if next time it happens and see if it releases.

 

Man, it sure would be nice if O.P. followed up on this!

 

I doubt that is the issue. Lack of vacuum results in a very hard pedal that takes significant effort to push and stop the vehicle. It would not affect the brake release as that is controlled by the fluid hydraulic pressure (or mehanical friction in the calipers).