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Hissing sound when the brake pedal is pressed?

1990Pioneer

NAXJA Forum User
Location
Denver, Colorado
Hello all, Ive noticed that when I step on the brake pedal it hisses, sounds like an air leak? It makes this sound when I step down just a bit, then it will stop if I step down harder. then if pressed down all the way, it will start hissing again. Any ideas, thanks again!
 
hundred bucks (give-or-take) for a reman. unit at the local parts house.

I'd check carefully for brake fluid in the booster before replacing just the booster, though. If the master cylinder has a leak in the back of it, you will kill the new one again in short order.
 
Could be normal. The way power brakes work is that there is a rubber diaphragm in the big metal drum behind the master cylinder. In normal driving operation (not braking), vacuum pressure is applied to both sides of the diaphragm. When you step on the brakes, the cabin side of the diaphragm vents to atmosphere, which causes that side to equalize to normal air pressure. Meanwhile the engine side is still under vacuum, so the diaphragm gets pulled that way. That is the power brake assist--a big diaphragm under engine vacuum gets pulled towards the engine, which helps you to push the brake rod through the master cylinder.

Sometimes you will hear the whoosh noise when you step on the pedal, which is air rushing into the cabin side of the diaphragm. If you hear a constant noise when the pedal is down, then you might have a tear or hole in the diaphragm, allowing the engine vacuum to leak into the cabin side.

Do your brakes work alright or are they really hard? Do you hear the noise always or just when pressing the pedal?
 
Do your brakes work alright or are they really hard? Do you hear the noise always or just when pressing the pedal?
.

I agree. If you have a leaking diagram then you will have a much heavier pedal pressure to stop. If it is stopping fine without having to stand on it then I is probably fine.
 
Sometimes the foam silencer inside the booster's pushrod hole (which is where it vents from when the pedal is depressed) wears out or degrades allowing the noise to become louder. It is not a replacement part, nor does it affect the operation of the booster. If you can tolerate the noise and the booster operates normally, leave it be. If there is any sign of improper operation, change it.
 
It only makes the sound when I press the pedal. The brakes work just fine, I dont have to put much effort into stopping. When I push down the pedal it will make the sound constantly until I press it to the floor then it stops.
 
I have found that to be an early warning sign that the booster is on its last legs. Sometimes it takes a good while for them to go completely bad (even a year or two), other times if the brake fluid is leaking in the booster area, it fails much faster.

Check for leaking, missing brake fluid in the MC, that never shows up anywhere!!!! Keep a close eye on it, until the brake performance is an issue.

If the vacuum leak gets bad enough it will cause a high idle at idle with the brakes applied.
 
I have found that to be an early warning sign that the booster is on its last legs. Sometimes it takes a good while for them to go completely bad (even a year or two), other times if the brake fluid is leaking in the booster area, it fails much faster.

Check for leaking, missing brake fluid in the MC, that never shows up anywhere!!!! Keep a close eye on it, until the brake performance is an issue.

If the vacuum leak gets bad enough it will cause a high idle at idle with the brakes applied.

digging up an old thread......

I have replaced my master cylinder, and now I get the high idle when I push the brakes, and the brakes literally don't stop the vehicle. Could this mean I ripped a hole in the brake booster diaphram?
Is there a way to check the diaphram without removing the whole booster?
 
digging up an old thread......

I have replaced my master cylinder, and now I get the high idle when I push the brakes, and the brakes literally don't stop the vehicle. Could this mean I ripped a hole in the brake booster diaphram?
Is there a way to check the diaphram without removing the whole booster?


The problem may have nothing to with the new Master Cylinder or old diaphragm.



Are you getting a hissing sound?



It could any number of other reasons for the high idle.


But the Jeep in park, at idle, cold and later warmed up, try cold and hot states.


Press the brake (while in park) and see if the Idle jumps.


If it does run up the idle, disconnect the large rubber vacuum hose attached to the Vacuum Booster and plug the hose end so air can NOT get sucked into the intake manifold.



If the High idle problem stops, it is the vacuum booster.



If the high idle does not stop it is wiring, connectors or sensors, like the brake switch under the dash not telling the AW4 transmission computer to unlock the Torque converter, or a ground issue in the wiring...or TPS Throttle Position sensor or or ground on it.


Tell us what your find please. Good hunting
 
Thanks. I will plug the booster and see what happens.


Actually you need to remove the Vacuum hose off of the nipple on brake Booster check valve and Plug the open hose end. Other wise the engine will race way up, way too high, which dare I say it "SUCKS"? LOL
:laugh3:
 
The problem may have nothing to with the new Master Cylinder or old diaphragm.



Are you getting a hissing sound?



It could any number of other reasons for the high idle.


But the Jeep in park, at idle, cold and later warmed up, try cold and hot states.


Press the brake (while in park) and see if the Idle jumps.


If it does run up the idle, disconnect the large rubber vacuum hose attached to the Vacuum Booster and plug the hose end so air can NOT get sucked into the intake manifold.



If the High idle problem stops, it is the vacuum booster.



If the high idle does not stop it is wiring, connectors or sensors, like the brake switch under the dash not telling the AW4 transmission computer to unlock the Torque converter, or a ground issue in the wiring...or TPS Throttle Position sensor or or ground on it.


Tell us what your find please. Good hunting

Good directions, kudos:thumbup:
 
Actually you need to remove the Vacuum hose off of the nipple on brake Booster check valve and Plug the open hose end. Other wise the engine will race way up, way too high, which dare I say it "SUCKS"? LOL
:laugh3:


Unplug booster vacuum line.
Plug check valve on booster
Turn Jeep on.


High idle. Constant. Nothing changes when I push the brakes.
Connect the brake booster and the idle drops to normal, and when I push the brake pedal the high idle returns, as expected.

So vacuum booster is the problem, correct? Needs replacement I assume.
 
I must have missed something when I installed the master cylinder. I took it off, saw no new brake fluid, reinstalled it and now everything works as before. Maybe I didn’t get the pushrod from the brake pedal lined up with the master cylinder. It’s the only thing I can think of.
 
........Maybe I didn’t get the pushrod from the brake pedal lined up with the master cylinder. It’s the only thing I can think of.
Or the pushrod wasn't fully seated (or not centered) in the booster.

Anyway, glad you got it fixed!:cheers:
 
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