Brake hiss

1994 GMC Suburban C1500 2WD. With engine running, brake pedal area has hiss when I press on the brake pedal, can hear it underneath and in engine compartment.

Is this the sign of a bad brake pedal booster? Is there any seal between the booster and the master cylinder that can cause this?

Unrelated question: anyone ever have a master cylinder for a GM vehicle where brake fluid leaked between the brake reservoir and the master cylinder itself? I've had several go bad there, first one was replaced by a mechanic, all with DOT 3 or DOT 4 fluid. thanks.

"1994 GMC Suburban C1500 2WD. With engine running, brake pedal area has hiss when I press on the brake pedal, can hear it underneath and in engine compartment. Is this the sign of a bad brake pedal booster? "

I changed out the booster today, $87 from AutoZone, lifetime warranty, and the hiss is gone. I met up with some "interesting" typical GM engineering with it: clip in rod that passes through the booster shaft hole was tough to get to, hidden and cramped area, the studs on the booster were twice as long as need be, and special GM nuts couldn't be turned by hand even after loosenting, and the booster rod was in the way of turning the ratchet anyway. I used a deep 5/8 inch socket (3/8 drive), one click at a time, took like 15-20 minutes to get each nut off. I bought a 15mm deep socket from AutoZone, and bought four replacement 10mm x 1.5mm flange nuts at Ace Hardware, and four wave lockwashers so I could hand-thread those nuts most of the way down to make installation easier (which turned out to be a GREAT idea).

"Unrelated question: anyone ever have a master cylinder for a GM vehicle where brake fluid leaked between the brake reservoir and the master cylinder itself? I've had several go bad there, first one was replaced by a mechanic, all with DOT 3 or DOT 4 fluid."

I also picked up a free replacement master cylinder from CarQuest and installed that as well. Because of the leaks I've had in the past, I tried not to rock the reservoir whenever I took the reservoir cap off during the bleeding process. And why is it such a pain to line up the brake fittings on this vehicle with the master cylinder to avoid crossthreading?

yes it's your brake booster that's making the hiss sound. check the hose comming to the motor to the booster and see if it's leaking but sounds like going to need a booster here shortly if not the hose? when you go to stop one time and the brake pedal will be hard as a rock you'll know it's the booster..

yes it's your brake booster that's making the hiss sound. check the hose comming to the motor to the booster and see if it's leaking but sounds like going to need a booster here shortly if not the hose? when you go to stop one time and the brake pedal will be hard as a rock you'll know it's the booster..

that should be the problem on that hiss noise...but the brake pedal should be hard to depress...also the engine idle usually jumps/dips...when braking...

the booster can be damaged if brake fluid contacts it ....

Replaced the booster, bitch to get to the four attaching nuts, works fine now. There was no leak at the rear end of the master cylinder (ever), only leaks I ever had were at reservoit-to-cylinder seals.

is it wise to use one from a junk yard?:licka:

is it wise to use one from a junk yard?:licka:

At $87 new, with warranty, and the hassle of the install, figuring used one would might be $10-$20, you need to decide if it's worth it. Personally, I wouldn't do used; but if you have more time than money, your call. Maybe you could use a hand vacuum pump to test such a used one????

is it wise to use one from a junk yard?:licka:

no,,,the reason is the rubber after a few years goes bad,,,best get new...

if it is brake hardware like drums/rotors even calipers, and your on a small budget I have never had a problem,,,but anything that has rubber would not be a good idea...the internal rubber diaphram is what fails ,,and it takes only a very small leak to cause these type of problems..

the easyiest way to get off fthe nut is to use a deep socket, every long extension and swivel head. it takes about 10 mins to take off.