Technical Brake booster problem?

I am having a problem with my brakes, but not sure if its the booster. I have an aftermarket dual diaphragm booster on my truck (s10 ad truck frame swap). The brakes work ok ,meaning it stops ,but will not lock them up. However on occasion when it is damp out and the truck sat the night (outside) it will give me 1 or 2 great stops . I also notice that if I turn the truck off and hit the brakes the booster won't keep a soft pedal for more than 1 stop. Even that 1 stop isn't the greatest.
I am really leaning toward a booster problem ,but want to be sure. Outside of buying some gauges to check brake pressure output any thing else I can do? Anybody ever have this before with the damp weather?
Thanks

 

what type of motor? where is booster getting vacuum from? post pics

 

It has a tune port small block Chevy. Getting vacuum from the back of the manifold. Will try and post pics.

 

I will add that I have about 17" of vacum.
Thanks

 

I've only had experience with two aftermarket dual diaphragm boosters both were 8 inch from MPB. They were both garbage and worked like you say yours is. They also had around 17" vacumn

 

To determine if the booster is "boosting" pump the brake pedal several times with motor off, put moderate pressure on brake pedal and start the car, if the pedal "drops " some upon start up the booster is working....also when you get 1or2food stops when the linings are damp it is a lining issue (too hard perhaps) or a pressure issue, maybe too large a master cylnder bore..

 

The booster acts like it is working. The pedal drops when you start the car. The master cylinder is the s10 one that came off the donor car.
I am leaning towards the booster because as phat rat stated these are garbage. I had one before that acted like it worked ( pedal drop test) . It just didn't have enough to stop the car good. After some discussion with the seller they gave me another and it worked fine.
Thanks

 

went thru the same thing. New booster, 18" of vacuum, pedal would drop like booster was working-just would not stop as it should. Exchanged for another new one-works as it should-go figure.

 

Boosters don't stop the car, the brakes and most importantly the master cylinder do that. If the brakes aren't right, you can put any size booster you want on the car it won't make any difference.

You stated your brakes won't lock up, sounds like a good thing to me.

 

57 heap, I disagree. All three have to work together or you won't stop good. If you have power brakes and the booster isn't working correctly you don't have the braking power you would if it wasn't in the system. As I said earlier, I had two aftermarket boosters that were garbage. The second one was a replacement for the first and it didn't work any better. I replaced them with one from an junkyard Astrovan which is still working fine 13 years later. BobK had the same experience with his booster from the same company and after driving mine and seeing how good the brakes worked he did the same as I did.

 

These aftermarket boosters seem to be hit or miss. I think I have a miss with this one.

 

I figure I would give an update.
I put on 2 different boosters. Both came from different year camaros I had in the garage 80's and 90's. I put new front rotors,pads and calipers.
I still can't get a good panick stop. It takes to long I feel to stop ( it can't lock up the brakes).

I checked fluid pressure at the booster and got about 1200-1300 psi at the front and rear. I checked after the proportioning valve and got 12-1300 for the front and about 6-700 for the rear.
I am running the stock proportioning valve from the 82 s10. I also have the same master cylinder (step bore ) and the metric low drag calipers.
I am running 16 " steel wheels on this that I believe are 6" wide. I am running a much bigger wheel and tire than the s10 had.
I am down to thinking either the calipers and 10-1/2 " rotors just aren't enough to stop it the way I feel it should stop.
Next step is to just eliminate the proportioning valve and see if that does any better.

Any one else have any thoughts?

Thanks

 

Adjustable proportioning valve? If so set all pressure to front and try a few panic stops( in a parking lot or safe area obviously) if it's better it's not the rotors or calipers possibly master cylinder diameter is not enough. Not sure if you have replaced it, built your own system or what other variables, just a couple thoughts .02 cents if you will

 

Not adjustable. It's the stock 1982 s10 valve along with the brakes. The only thing not original from the s10 is the booster.

 

I gave up on the booster game and just went with a small bore master in the end.

 

1200 -1300 psi should be plenty of pressure, what kind of pads are you using? Maybe you should get the glaze knocked off of your rotors and try a set of performance grade pads.....there is a HUGE difference in friction material and also how they are broken in.....