Brake issues
 

So I have a 78 gmc. New brake booster (60 miles on it but about a year and a half old still in the building process), new master, new everything. Was fine when installed initially. Then the brakes were hard to push and stop the truck. Long story but one engine later it’s finally ready to test drive again. When I start the truck the brake pedal sucks down all by it self. Nothing is pushing on it. I’m not even in the truck. I didn’t notice it doing this when it ran last but I bet it was. I was too worried about the low oil pressure to worry too much about the brakes. I’m thinking the brake booster has failed. If you pull up on the pedal and release it it sucks back down by it self. There is free travel before the push rod starts pushing on the master cylinder. Not sure what else it can be. Any ideas?

Re: Brake issues
 

Your symptoms are usually caused by a hole in the booster diaphragm allowing vacuum to the backside of the booster/diaphragm and causing the pedal to "stick" or suck down. Replacing the booster should cure your problem.