Hey everyone. I guess alot of you know about my brake problems (see "Brakes failed on highway") lately, but I don't think this is related to the hydraulic part of the system and I want to ask this on a separate post. I just want to ask if some signs I am noticing are signs of a bad brake booster (plus some additional info to rule out other stuff):

-Soft Hiss/air sound from under the dash upon brake application

-Been doing that for a while

-Pedal stops about 1" above where the gas pedal is at idle and car stops normally, but not so fast. I can slide my toe under the brake pedal with it still on the gas pedal in it's idle position.

-Car was driven by several shops after the master cylinder and caliper replacements and they said nothing about it nor did they mention the hiss noise.

-I still get about 4-5 brake applications after the engine is shut down until the pedal gets hard to press.

-I can press the pedal 4-5 times to get rid of the vacuum and then hold the pedal down as I crank the engine and it will fall away from my foot and go to it's full applied position under my foot.

-Pedal DOES NOT go to the floor or sink to the floor upon brake application. Warning light does not go on unless the parking brake is set (normal).

-Brake fluid is at the Max level on the reservoir and only fluctuates + or - .5 mm.

 

Yeah, I'm pretty sure your brake booster is bad. If you want to make sure
Run the engine. Have someone step on the brake. When they do, and the engine runs rough, then pinch off the vacuum line going to the brake booster. If the engine runs smooth again, that's your problem.

you might also check out this thread

https://blazerforum.com/forum/showth...=brake+booster

 

What do you mean by "rough"? It seems to run fine no matter what. If anything, it runs quiet and like a Swiss watch. Idles at 550-900 rpm depending on how warm the engine is, no check engine lights, no funny noises, it seems to run fine even when I hit the brakes. Hitting the brake seems to have no bearing on how the engine runs. The only thing I did notice on the gauge cluster is that the voltmeter dipped like 1/4-1/2 a notch when the brakes were applied (because of the brake lights). How much is it usually to replace the brake booster and how hard is it? Is there anything else that could cause this like the check valve? I just don't want to get the brake booster replaced and find out that it did not solve the problem like what happened with the master cylinder last month because that unnecessary fix turned a simple caliper failure into a money pit because it doubled the repair costs. I just want to make sure that is the problem before I call the shop. Thanks for your help.

EDIT: Does the hiss alone mean that there is a problem? That seems to be the only real abnormality here. It only hisses when the brake pedal is being pressed down and it stops when the brake pedal stops moving. It doesn't do it while the pedal is down, on the way up or up. Is that normal or is that a problem?

 

If your brake booster was bad, it'd hiss the entire time you were holding the pedal down as air would be coming in the vacuum leak. The engine would run rough, too, since it *is* a vacuum leak.

 

It isn't running rough and the hiss stops when the pedal is down, so maybe it isn't the booster? I really do hope it isn't because I was just quoted $168 for the booster and $280.00 for 5 hours labor. I sent an email to a repair shop last night and I got a reply. It is possible I have a bad hose or a check valve external to the brake booster that isn't something that you can easily see? Thanks.

 

Seems like it could be possible

 

Is there a chance that the pushrod was not readjusted when they replaced the master cylinder unnecessarily?

 

Ok, I am bringing the truck in tomorrow to get it checked out, but now I have a new problem. The past 2 days when I go to crank the engine for the first time after it sitting for 24 hours in hot weather, I hear a sound similar to the exhaust being open (as if the engine was running with no exhaust system) for like one second and it seems to be coming from the left side under the hood. It does it for like 2 starts after that, but then it doesn't happen until I let it sit again for overnight. Still a chance the booster is bad? Here are the facts as of now.

-Faint (can't hear it when the radio or a/c is on) air sound when I am pressing the pedal to stop the truck. NOT PRESENT when the pedal is down or when it is up or on the way up. I heard on some sites that a faint air sound is normal, but I am having it checked because I don't want to take any chances. It has been doing that for a while and since other cars I have driven has had the same hiss, I originally thought nothing of it, but since I have had other brake problems, I feel it warrants attention.

-I have to press the pedal like an inch before I get any braking action.

-Master cylinder was replaced a month ago, but the cause of 2 brake failures 200 miles and 2 days apart was found to be a rear caliper that got stuck and the pad wore all the way down until the friction cause enough heat to boil the brake fluid to the point where the brakes failed. There could be a chance that it was not replaced properly because the shop replaced it after the first brake failure and the shop manager in NJ got argumentative when the brakes failed and then fixed themselves the 2nd time.

-Pedal does not go to the floor, no warning lights and the pedal stops like a 1/2-1" above the gas pedal at idle position.

-The first 3 brake applications after I shut the engine down the pedal goes down to the same general position and then it hardens at the 4th application then on the 5th or 6th I have no more reserve.

-The engine runs fine no matter where the brake pedal is. No rough running or check engine lights (or any warning lights for that matter). It idles smooth at 500-550 in gear and 550-600 in park or neutral.

-Pedal (when the reserve is gone) falls away from my foot normally when the engine is started. It does not stay hard when I crank the engine.

-Brake fluid is fine. No leaks.

If after they test it the booster is confirmed to be bad, how bad of a repair am I looking at? Is a brake booster an easy item to replace? I already know that alot of stuff will have to be removed to get to it, but is it really 5 hours labor? Should I have the check valve replaced either way if it is easy and cheap enough? Thanks

EDIT: After further research, I am going to get the check valve replaced no matter what because of the mileage.

 

Truck is at the shop now. All I got to do is wait for the phone call. Hopefully it is nothing.

 

Never mind on this post. It was nothing. I was told that they all make that noise and it is normal. The booster is known to be good.