Power Brake Experts Needed!!!!

[DANTIP]

My recently acquired '72 SWB 4x4 has factory power brakes but my pedal is "hard". My engine vacuum at idle is around 18", the check valve holds vacuum, my calipers aren't frozen-up, the booster looks new, my PCV valve is working properly. I have read some tests for brake boosters and mine seems to pass the test where with the engine off you pump the brakes a few times to use up vacuum, then hold the brake pedal down and start the engine. The pedal does go down a little bit, which should indicate a functional booster.

What am I missing?

[70STOVEBOLT]

I'm not an expert by any means, but have you checked the prop valve to make sure it is the correct type, and/or is working correctly? That's the only thing I don't see a reference
to in your post. Good luck.

Mark

[DANTIP]

Mark,

Thanks for your input! I didn't look closely at the proportioning valve but it looks like a factory part and the plunger in the front was working when I replaced my brake fluid.

Dan

[MARKDTN]

Could it have an inline check valve that is in backwards?

[MacGyver72]

I'm no expert either and I don't know exactly what you mean by "hard". My power brakes are hard too but that's a good thing. I've found that the pedal should go down about 1/4 to 1/3 of it's travel and basically lock up the wheels at that point. I'd be more concerned if they were soft and the pedal was going down slowly while sitting and holding the brake.

[DANTIP]

Originally Posted by MacGyver72

I'm no expert either and I don't know exactly what you mean by "hard". My power brakes are hard too but that's a good thing. I've found that the pedal should go down about 1/4 to 1/3 of it's travel and basically lock up the wheels at that point. I'd be more concerned if they were soft and the pedal was going down slowly while sitting and holding the brake.

The pedal requires too much effort to push it down to stop normally. It really acts like I have a bad booster, too little vacuum or something like that.

[MacGyver72]

Ahhh I understand now. Little slow on the uptake at times. Yes, you apparently have a bad booster since you seem to have good vacuum at the carb base. I don't know if it would help but you could experiment with taking the vacuum line off the carb, plug it and see if it changes your brake situation. If not, you probably have a booster issue.

[DT1]

My 71 just started to do that last weekend. The first thing I thought about was the booster going bad since the pedal wasn't mushy. I did some checking and found a small split in the vacuum line...Hello! I replaced the line and it still was no better. I checked the check valve and grommet...Good. Took it out for a drive and heard the rotor squeal ever so slightly so I pulled off the rear drums and found a leaking wheel cylinder. I replaced it and bled them off. Still no better so I replaced the booster. Still no good, so I completely blocked all the vacuum ports from the engine except to the booster. I still have a hard pedal and I'm completely out of ideas.
I'll bet by this point you thought I had a solution for you.

[DANTIP]

Originally Posted by DT1

My 71 just started to do that last weekend. The first thing I thought about was the booster going bad since the pedal wasn't mushy. I did some checking and found a small split in the vacuum line...Hello! I replaced the line and it still was no better. I checked the check valve and grommet...Good. Took it out for a drive and heard the rotor squeal ever so slightly so I pulled off the rear drums and found a leaking wheel cylinder. I replaced it and bled them off. Still no better so I replaced the booster. Still no good, so I completely blocked all the vacuum ports from the engine except to the booster. I still have a hard pedal and I'm completely out of ideas.
I'll bet by this point you thought I had a solution for you.

Please let me know if/when you find the solution (this sounds very similar)!!!

[DT1]

Originally Posted by DANTIP

Please let me know if/when you find the solution (this sounds very similar)!!!

Update
I just finished rebuilding my entire brake system. I replaced the vacuum booster a couple of weeks ago and today I replaced the rotors, calipers, wheel cylinders, pads, proportioning valve, master cylinder, drums turned and shoes/pads. I completely flushed and bled the system. My brakes still feel the same (Hard Pedal, no mushy).
I'm beginning to wonder if I might have got a bad booster.
Anyone else have any thoughts??

[DANTIP]

Originally Posted by DT1

Update
I just finished rebuilding my entire brake system. I replaced the vacuum booster a couple of weeks ago and today I replaced the rotors, calipers, wheel cylinders, pads, proportioning valve, master cylinder, drums turned and shoes/pads. I completely flushed and bled the system. My brakes still feel the same (Hard Pedal, no mushy).
I'm beginning to wonder if I might have got a bad booster.
Anyone else have any thoughts??

Have you checked your vacuum at idle? I am told that 18" is considered the minimum required for power brakes.

[DT1]

Originally Posted by DANTIP

Have you checked your vacuum at idle? I am told that 18" is considered the minimum required for power brakes.

The engine is fresh and at 2500 Ft elevation, mine is at 18in. The vacuum will vary with elevation. I also have no vacuum leaks. A few weeks ago I washed the truck and just after, I noticed the problem. The brakes have worked fine for many years so I figured it could be just wet rotors from the wash. A few minutes of driving told me different. I really think I just got a sub standard re manufactured booster from NAPA. I'm going to replace it next week and I'll let you know how it works out.

[71 super]

Dan swap out the booster it sounds to me like it has gone bad. I put a "good" booster on the 68 a year ago and it had a hard pedal, swapped to a new booster and now the pedal feels correct.

[DANTIP]

Any other thoughts? Brake pedal/linkage binding, internal master cylinder leak, wrong part installed, is 18" vacuum sufficient, etc.???????

[DT1]

Originally Posted by DANTIP

Any other thoughts? Brake pedal/linkage binding, internal master cylinder leak, wrong part installed, is 18" vacuum sufficient, etc.???????

18in of vacuum is more than sufficient.

[DT1]

Originally Posted by 71 super

Dan swap out the booster it sounds to me like it has gone bad. I put a "good" booster on the 68 a year ago and it had a hard pedal, swapped to a new booster and now the pedal feels correct.

As coincidental as this may sound, it has happened to me more than once with re-manufactured parts.

[ls1nova71]

Definitely sounds like a booster problem. Since you said" newly acquired" I'm going to assume it has done this since you've had it and is not a new problem? I would try another booster, preferably a known good one (original GM if you have one) I've seen bad remans.and aftermarket (China junk) right out of the box.
Posted via Mobile Device

[DANTIP]

Originally Posted by ls1nova71

Definitely sounds like a booster problem. Since you said" newly acquired" I'm going to assume it has done this since you've had it and is not a new problem? I would try another booster, preferably a known good one (original GM if you have one) I've seen bad remans.and aftermarket (China junk) right out of the box.
Posted via Mobile Device

Yes, the problem came with the truck. I think I'd have to press REALLY hard to get the tires to lock up. I do have other trucks that I could borrow the booster from. It may just be worth my time/$$ to order a new booster/master cylinder and install it.

[Hart_Rod]

Pull the vaccuum off of the booster (make sure you plug the leak, ). If it still feels the same, then you have a booster problem.....

[DT1]

Originally Posted by Hart_Rod

Pull the vaccuum off of the booster (make sure you plug the leak, ). If it still feels the same, then you have a booster problem.....

In my case you can tell a big difference between vacuum or no vacuum.
The symptom is a brake pedal that requires more force than usual to apply the brakes. It almost feels like the shoes or pads are glazed.
This weekend I'm going to rebuild the calipers, install new pads and shoes and flush the entire system.

[DANTIP]

Is it possible to use a hand-held vacuum pump to determine if inadequate vacuum may be preventing the booster from working? My thought would be to remove/plug the vacuum line from the manifold and plug my vacuum pump into the hose that leads to the booster check valve. I would then pump the pressure down to say 25" then try the power brakes. If the pedal feels more "normal", I would have more confidence that I should concentrate of finding a potential vacuum leak rather than changing brake parts.

Any thoughts?

[DT1]

OK, Today I replaced the booster I got a couple of weeks ago with another re-manufactured unit. I'm happy to state that the brakes are as good as new.

[DANTIP]

Originally Posted by DT1

OK, Today I replaced the booster I got a couple of weeks ago with another re-manufactured unit. I'm happy to state that the brakes are as good as new.

That's Good News!! Thanks for following up with the fix!

[snacks]

with the motor running take some needle nose pliers and pinch the vaccume line off,your idle may drop alot,meaning internal vaccume leak replace booster

[DT1]

Originally Posted by snacks

with the motor running take some needle nose pliers and pinch the vaccume line off,your idle may drop alot,meaning internal vaccume leak replace booster

It makes a lot of sense and that's what I did. The vacuum remained the same and that's why I was having trouble blaming the booster.