Ok, so on my 71 vette with factory power brakes, everything is new including booster and master cyl, hard lines, proportioning valve/switch, flex lines, Wilwood D86 and 4 calipers, and new rotors.
Ls swap, first run after everything bled and vacuum connected motor running, the brake pedal felt lots of assist and would lock the brakes down with a hard pedal and would release slowly within ten or twenty seconds. With vacuum disconnected brakes worked normal but with a strong push with the brake leg. Chased pushrod lengths and pedal adjustments, and decided it must be the master/servo combo even though they were bought as a set, and both are for a shallow master piston. Didn't feel good about the pushrod length at the master. So I got this new cast aluminum but factory type master from cpp and I really like it, and it bench bled better than the cast iron one for whatever reason. Also, the pushrod length is now about dead on perfect with about a 16th clearance maybe less. The pedal pushrod has about 1/8" play before contacting the booster piston. And of course the brakes are still about the same. But the assist is pretty strong now and any slight press of the brake pedal and it just goes down to full on hard pedal and locked brakes. Almost like the booster takes the pedal out from under your foot. Have to disconnect vacuum to release brakes. Again with vacuum disconnected brakes work normal with a good leg workout and a little better than before maybe because of a better bench bleed.
So I've put off pulling the vacuum booster for obvious reasons and I'm down to decision time. Last thing I want to do is go through changing the booster to find I still have the problem. I keep thinking it could be an adjustment issue but really I don't think so.
I'd love to get some feedback, words of wisdom from you guys before I jump in the ring with this booster. Any ideas?

ps... the one way valve on the booster is new and working fine.

 

Do the brakes release if you lift the pedal with your toe? Sounds like a booster problem to me.

 

Here is my two cents worth, but its only worth a penny:
Whats the condition of the brake pedal spring under the dash?

 

No it's pinned down tight

 

New spring, completely restored pedal box. And the pedal won't pull up.

 

Booster, even if it somehow started working right, I'd never trust it.

 

Yes and at this point I can't see how it could be anything else. I wish there was a slightly smaller diameter unit I could use. Even a quarter inch would get a bit of clearance between the clutch master and the booster. Stainless would be even better!

 

I went with hydra-boost, takes up a lot less room, but has hoses added.

 

I've been tossing that one around too, prob the last chance for me to do it.

 

All new parts and it didnt work??. I would be returning them.

 

Is there a way to pretest the Booster before install ? I just removed my 1970 Booster as I'm rebuilding the car and I must say "I do not want to redo a removal down the road". Just looked into HyraTechBraking.com it's calling my name I think.
Good luck with your work, I feel your aggravation regarding having to redo something. PIA !

 

Sure sounds like the return spring in the booster that closes the air valve is malfunctioning. I don't think there is anyway to bench test it.

 

David you're right I think. Changed the booster Sat. Brakes seem to work normal with the short fire up and test with no drivers seat. But felt normal. Just bad luck with a defective new booster.
Job wasn't as bad as I was making it, pull the drivers seat is a must though, rest is just a few beers...
Bill

 

Bought the booster two years ago, too long resto-mod project you know. Out of warranty, gave ZIP a chance to make it good but no go.