How to test power brake booster

[Gene]

I'm slowly trying to put my truck back together, and I have bled the brakes and bench bled the master cylinder, and it feels like I have a good firm pedal but I have to put alot of pressure on them to stop? I dont hear any noise coming from the booster like some i've heard. The brakes were working fine before I took the truck apart? I checked vaccum at the manifold and i'm reading 15 in. Hg.

[Gene]

Forgot to say, i'm using a stock 71 booster and master cyl. to power 91 brakes on 91 differentials, I dont think this has anything to do with the problems but you never know?

[treveiger]

Interested also

[stich626]

hand held vac pump. it should hold vac.

[Gene]

Originally Posted by stich626

hand held vac pump. it should hold vac.

I've got a handheld vaccum brake bleeder, that should work right? Well, I hooked it up and it wont hold any vaccum at all. So whats my next step? Is there any repairing my old one or just replace it?

[Gene]

I also checked another later model booster that I had laying around, it wont hold vaccum either? What are the chances that I have 2 bad boosters?

[stich626]

iirc you have to push on the pedal then pump.the vac pump..

[Gene]

Originally Posted by stich626

iirc you have to push on the pedal then pump.the vac pump..

Ok, hooked up the pump and pushed on the pedal and still not holding vaccum at all.

[hayhauler71]

The booster should hold vacuum with out pushing the brake pedal. Make sure your vac pump has a good seal and it will take more than a few pumps to pull vacuum in a booster. Also make sure your vac pump will hold vacuum.

[Gene]

Originally Posted by hayhauler71

The booster should hold vacuum with out pushing the brake pedal. Make sure your vac pump has a good seal and it will take more than a few pumps to pull vacuum in a booster. Also make sure your vac pump will hold vacuum.

I know the pump is good, I used it to bleed my brakes and it held good vaccum during the process. So, exactly what goes bad in the booster.

[hayhauler71]

Originally Posted by Gene

I know the pump is good, I used it to bleed my brakes and it held good vaccum during the process. So, exactly what goes bad in the booster.

The rubber diaphragm inside.

[ls1nova71]

With the engine off, pump the brake pedal to remove all vacuum, the pedal should be hard. Then push the pedal down and start the engine, if the pedal goes down, it's working, if it doesn't, it's bad.