Hi All... I have a good firm pedal when the engine is off, but when i start the truck and touch the brake pedal, it goes to the floor and stays there and the brakes are locked up... i can manually release them by pulling the brake pedal up, but obviously something is wrong.

background: recently installed the 6 piston disc brake willwood conversion on my 67... along with it i put a willwood disc/drum master, a 7" dual diaphragm booster, all new lines. deleted the old distribution block (because the willwood proportioning valve serves that purpose, and replaced everything with factory components in the rear. there are only 2 reused brake components in this whole system... the literal brake pedal and the dust shields/backers on the rear drums.

at first i though maybe the booster wasn't properly adjusted for the master, but i have just shy of a 1/16" between the plunger and where it touches the spud in the master. then i thought maybe the push rod is binding, but it's completely straight going it. then i thought maybe its adjusted to always have some pressure on the booster, but it's not. I re-bleed everything extremely thoroughly, just to rule that out and nothing changes. it feels like very good manual brakes with the engine off/disconnecting the vacuum from the booster. it has to be a bad booster right? any thoughts/advise would be greatly appreciated.

 

Yes the booster is the issue. There is a valve inside the booster that should control the balance of vacuum and pressure on each side of the diaphragm. That valve is there to prevent this exact scenario. It regulates the boost effect based on pedal travel. Sounds like it's stuck.

 

thanks driveway... i know they make defective parts all the time, but i always feel like it's more likely i messed something up lol

 

it was absolutely the brake booster... got a new one in and it works. already miss the feel of the manual brakes, but as long as the wife is happy i guess