Brake Booster issue

[redfj]

So except for the brake booster and brake hard lines, I've replaced or rebuilt every part of the brake system on my '71. Everything is rebuilt to stock with drum brakes all around. Here are the symptoms:

Pedal will go to the floor the first pump or two and then everything is good for the rest of the drive. I know this sounds like a bleeding problem, but I've bled the brakes so many times I've lost count. I even had a shop do it since I thought I was going crazy. No leaks anywhere that I can tell. Also, this is intermittent and sometimes the brakes work fine the entire drive.

I did some tests to try to rule out the brake booster by pumping up the brakes until they're hard with the engine turned off. Then I started the engine and the brake pedal stayed right where it was. From what I've read I would have thought it would drop a couple inches once the engine turned on. Is this definitely a booster issue or am I looking at a vacuum issue? I've also replaced the check valve to be safe.

I'm tempted to just get a new booster and hopefully be done with it, but if there are some more things I can check to isolate this issue, I'm all ears.

 

[71FFJ40]

Can you hear a hissing sound at the booster?
Have you used a vacuum gauge?How many inches of vacuum?

 

[redfj]

I guess that needs to be my next test... What should I expect for normal vacuum pressure?

 

[Curt]

Booster failure is usually a hard pedal not soft, you replaced the master then? You should be looking at about 18 inches of vacuum at idle.

 

[redfj]

Yeah I replaced the master. And I wouldn't be surprised if the old master leaked into the booster.

Looks like I'll check vacuum pressure and probably get a new booster. Thanks guys

 

[DustyFJ]

Are the front brake cylinders adjusted properly? It sounds like they need to be adjusted tighter a few clicks???

 

[redfj]

Are the front brake cylinders adjusted properly? It sounds like they need to be adjusted tighter a few clicks???

I've adjusted them a lot, but I suppose I could look at them again. Any reason why you mention the front and not rear?

 

[DustyFJ]

If I remember, and I'm probably wrong, aren't the rears self adjusting if they are working properly? The fronts aren't. I've since swapped to disks up front but when I was running drums I'd have to pump the brakes to get a firm pedal when the fronts were out of adjustment.

 

[numby]

If you replaced the master (my first thought) then adjusting the brakes is a good call. You may be just taking up the slack with the pedal pumping. I don't think any pre '78 cruiser has self adjusting drum brakes either front or rear. I could be wrong. Wouldn't be the first time.

 

[ibinazbass]

This, in my mind, sounds like a master cylinder issue; so here are a few questions regarding that.

Did you rebuild your old one or buy new/reconditioned?
Who's master cylinder did you use?
Did you bench bleed your master?

If the drum brakes were out of adjustment then you would have to pump the brakes at every stop (I have a 1968 with the single channel master with 4 wheel drums, I speak from experience on this one!). The springs return the plungers to fully compressed as soon as you let of the brakes. There could still be some trapped air in the master cylinder causing this issue too. Just my .02