Brake booster problem

[panhead59]

The vac fitting that goes into the front of my power brake booster broke so i just replaced it. Backed out of garage and as soon as i touched the brake pedal, brakes locked up. Never had or heard of this before. Thoughts ? Brakes stayed locked up. I did apply brakes without vac so maybe that proportioning valve got "misaligned". How to fix ? Its a CPP booster and valve.

[Mike (Asheville)]

The booster itself has nothing to do with the proportioning valve, only the amount of assist you get when depressing the pedal. It sounds more like a bad booster. Are the brakes locked up even after shutting off the engine? How about after removing the check valve and letting air into the booster?

[panhead59]

All new components. Installed and working for 6 months or so, truck not completely finished but brakes have been working. Still locked up after engine is off.

[Mike (Asheville)]

Maybe the pushrod linkage behind the booster is binding?

[panhead59]

Problem solved. Your mention of a check valve got me looking for one. Nothing on the booster except the vac fitting that broke ( actually just the round pc. On the fitting came off/lost ) . On the front of a cpp valve there is what appears to be a check valve. Its a fitting with a tiny "check valve" in the middle. Push it in and u get a tiny amount of brake fluid. That did it ! Learn something everyday. Operating brakes with open (broke fitting) must've "froze" the cylinder in proportioning valve. Seems odd but that's what happened. Thanks for info, again a very very helpful site with good people willing to chime in and help !

[panhead59]

Any further explanation as to what happened inside valve would be a interesting read. Tks.

[El Dorado Jim]

Originally Posted by panhead59

Any further explanation as to what happened inside valve would be a interesting read. Tks.

yeah, id be interested in what happened too

[68 TT]

Originally Posted by panhead59

Problem solved. Your mention of a check valve got me looking for one. Nothing on the booster except the vac fitting that broke ( actually just the round pc. On the fitting came off/lost ) . On the front of a cpp valve there is what appears to be a check valve. Its a fitting with a tiny "check valve" in the middle. Push it in and u get a tiny amount of brake fluid. That did it ! Learn something everyday. Operating brakes with open (broke fitting) must've "froze" the cylinder in proportioning valve. Seems odd but that's what happened. Thanks for info, again a very very helpful site with good people willing to chime in and help !

Glad you got it fixed. Good to know about the safety check valve. Is that behind the little rubber cap on the end of the proportioning valve?

I had that same vacuum fitting on my 70 Chevelle fail. The top come off when I was out on the road. It is not fun to stop a 3900 pound car with no power assist when the entire braking system was designed to work with it from the push rod connection point on the brake pedal all the way down to the master cylinder bore size and you are expecting the brakes to work like normal. I was literally pushing down on the brake pedal with both feet and pulling up on the steering wheel in an attempt to stop for a red light the first time I tried to stop after it came apart Luckily the piece was sitting in the engine compartment still when I stopped to check things out and I was able to glue it back together to get home.

It was the original GM part in my case though. I always check them out now on every rig I buy and add some model cement to the rim if they are that two-piece design and not the heat welded design that is more common. I have found quite a few that were pretty loose and probably a nice source of a vacuum leak.