Got a brake issue. When I put the 2+2 together the new booster I had sitting around didnt hold a vacuum. Plugged the vacuum line and drove it as manual disc brakes and they worked fine. I found another booster that holds a vacuum and installed it today. Now when I depress the brake pedal and release the pedal stays down and the brakes stay applied. I can pull the pedal up rather easily to release the brakes. Am I missing a return spring or is there another issue?
I'm no brake expert, and you haven't indicated whether you've got drum/drum or disc/drum. I'm guessing there should be a spring in the master cylinder to push the piston back. But, if you say it worked fine with the vacuum line plugged and in manual brake mode, then you've got a problem with the booster. I'm not sure what the check valve on the booster input should do, but maybe it's not working properly.
The car is a factory disc drum car. I reinstalled the original booster that was on the car when I dragged it out of the bush. No changes were made to the linkage geometry. I cant recall what I did with the master cylinder. I think I reused the master cylinder housing and installed the internals from a reman master cylinder.
Brake pedal returns fine with the vacuum line disconnected. Im of the same impression as per Clints buddy that there is a failed spring that cant overcome the engine vacuum once the pedal is depressed. Now the problem. Is it the booster, the master cylinder or is there a spring missing on the pedal assembly?
Try loosening the bolts that hold the booster to the firewall and see if that solves the brake hold. I ran into this a few years ago with an aftermarket booster. Ended up with a couple washers behind the booster to space it our far enough to allow the pushrod to fully release the brake pressure.
There is a valve and spring inside the booster. When you push the pedal it cuts vacuum to the other side of the booster. When you release the pedal the spring helps push the pedal back up. Maybe a broken booster spring or a bypassing valve ?
