I was told that my brake booster is going out, but reading some of the other problems I want to make sure it's not a vacuum leak. My car acts lik it wants to cut off and has actually cut off twice when I push the brakes. They are very hard with a hissing noise that sounds like it comes from the pedal.

before you go do this simple test
put the hand brake on and start the engine

idles ok --no problem
put your foot on the brake
idles ok ---problem to be found by reading the fault codes
idles rough or stops ---problem brake booster leaking vacuum==replace the brake booster
good units available from the wreckers/junk yards at a goo price

If the brake pedal goes hard right BEFORE the car dies, and not after, then you could have a problem INSIDE the brake power booster. try to verify this next time it dies at a stop, if you can safely pull the dead car off the road into a driveway (or try to make it stall in a bumpy parking lot) - then before you try to restart the car, open the hood, pull off the rubber hose where it's plugged into the brake power booster --> you SHOULD hear a big "Swoosh" of air being sucked into the booster because there's supposed to be a strong vacuum there even after the car stops running. if you get nothing there, your car is dying of an excessive vacuum leak through the brake booster.= bad booster.

Check for a vacuum leak somewhere. It could b the pcv valve on the valve cover or the hose going to the brake booster. Take the hose off the brake booster and plug it. then start the engine. It might also b the base gasket under the carb.

skdavisbob, This sounds like you have a problem with the brake booster that is attached to the firewall. When these boosters fail it is like trying to stop a house as you describe here. There is a one way check valve near the top of the booster that uses vacuum from the engine to operate. Inspect the hose and be sure that it is attached at both ends and that there are no leaks in this hose. This can be done by running the engine and listening for a leak. (hissing noise) Replace the hose if faulty. If you don't hear any vacuum leak then turn the engine off and pull the check valve out of the booster. This takes a little effort as it is quite secure in the grommet. You should hear a vacuum leak for a split second because when the valve is removed it will allow air in (if it is working properly) and you should hear the leak. If you don't hear the hiss, then test the valve by blowing through it both ways. As stated, this is a one way valve and you should be able to blow air through one way but not the other. If you can blow air both ways then the check valve is faulty. If the hose and valve are working properly then you will have to have the booster replaced. Good luck with this.

That sounds like a brake booster problem. The brake booster runs on engine vacuum to help make the brake pedal easier to push and easier to stop the tahoe. When a booster develops an internal leak, you can hear the vacuum escaping through the leak when you step on the brake. The engine stalls because the vacuum leak in the brake booster also causes a large vacuum leak for the engine.

To verify that this is actually the problem, go outside the vehicle with the hood open. Find the large brake fluid reservoir attached to a large round metal black (usually) canister on the driver's side of the firewall. (back of the engine compartment.) There is a large black rubber hose pushed into the top of the black canister, which is the vacuum line attached to the brake booster. Pull the rubber hose out of the grommet on the booster and stick your finger over the hose. Have someone start the tahoe. You will feel a considerable amount of vacuum pulling at your finger, and if you take your finger off the hose, the engine will stall. Also have someone step on the brakes while your finger is over the vacuum hose. If the noise is gone then you know the problem is in the brake booster.
Don't forget to re-attach the vacuum line to the brake booster. Failure to do this will result in the engine running very rough (if at all) and the brakes will require excessive effort to try to stop the tahoe.

Have you tested your brake booster and your calipers?Your brake booster doesn't make any noise, and it doesn't use any electricity or gasoline, but it ensures that you can stop your car with only a light touch of the brake pedal. Things weren't always like that: before the invention of the vacuum brake booster, cars still stopped. It's just that you had to really stomp on the brake pedal. The modern brake booster is an ingenious device that operates using something that your engine generates whenever it's running: vacuum. The brake booster takes engine vacuum via a rubber hose that runs from the intake manifold, and the brake booster uses that vacuum to amplify the pressure you put on the pedal. A light application of the brakes is translated by the brake booster into significantly more pressure on the brake master cylinder, ensuring that your car stops quickly. So what happens to the brake booster if your car stalls, resulting in a loss of engine vacuum? Early designers realized that gas engines were hardly foolproof, so they designed a little check valve into the brake booster circuit. The brake booster stores enough vacuum to provide full boost for two or three pedal applications even after the engine dies. The check valve on the brake booster is what keeps that vacuum from leaking out. And speaking of leaks, that's the reason most brake booster units have to be replaced. As your brake booster ages, the rubber seals and diaphragms that hold the vacuum tend to wear out and crack. If the brake booster can't hold vacuum (despite the check valve's best efforts), its time is up and you'll need a new or remanufactured new brake booster.

All right, if it was a loose brake booster hose, then it would cause the rough idle due to a large vacuum leak.

Replacing the hose would get rid of the hard braking as well, due to a loss of vacuum to the brake booster which uses vacuum force to add to the force of just pushing down the brake pedal.

So that would explain both problems.

Good luck on running your 1996 GMC Sonoma without anymore problems.

I would check the medium sized rubber hose that goes to the brake booster from the inlet manifold as it can swell internally and prevent vacuum getting to the power booster. Replace if necessary. I would also check all those pesky little rubber vacuum hoses that are used for pollution control. They tend to go hard and split where they connect to the metal piping if they are over 10 years old, allowing air / vacuum leaks into the inlet manifold. It could also mean that the rubber diaphragm inside the brake booster has developed a leak and causing loss of power assistance.

First inspect the vacuum check valve and the booster vacuum grommet for any leaks, even a pin hole in the check valve can also cause that to happen.

If the vacuum line to the booster is tight and has no air leaks, and no air leaks are in the vacuum check valve, then replace the booster unit.

just came across your posting, not sure if you have repaired the vehicle yet or not. just reading through this and it sounds like a faulty brake vacuum booster. the booster supplies power to the brake system by using engine vacuum