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Hissing Brake Booster (UTOH)??

wilcharl

NAXJA Forum User
Location
Wash DC
Guys my brake booster hisses when you apply the brakes you can hear it in the cabin... is it waht i think it is (New booster time) i wanted to do the upgrade ot the dual diaphram and change out the Master Cylinder but i really dont have time to do the plumbing so i am just gonna have to go with the single diaphramer but wanted to makes ure nothing else woudl cause the hissing during applicaiton other then the booster
 
the protective boot can hiss a little.

The test would be as follows:

1. with the engine off (preferably overnight) apple the brake make note of the sound and pressure.

2. with your jeep running apply the brake and make note of the sound and pressure needed to apply the brake

3. turn engine on run for 30 seconds do not apply the brake, then turn the engine off and wait one minute. Then apply brake and make note of the sound and pressure.

step 2. and 3. "brake applications" should sound and feel the same,

step 1. and 2. are meant for you to feel the difference between vacuum and non-vacuum conditions to see if it is just the normal boot hiss.

If step 3. feels stiffer than 2. it is likely your booster is not holding vacuum, because the booster should hold a vacuum for at least a few minutes after the engine is shut down. The factory does this so that you still have brakes if the engine fails.

booster replacement is not hard if you have a flexible back

hope this helps
 
very much so.
I think that mine is not holding vacuum. I had to kill the engine a few times while driving the other day, since the clutch went out and I had to start in 1st. As soon as the engine was off the pedal went hard, seems to work fine when the motors on. WIll have to check the little plastic check valve and all the hoses. Vaccum test was on the list for the weekend anyway, now along with clutch master.
 
Hissing booster can also mean a leaky Master cylinder...master cylinder leaks,brake fluid gets into booster,brake fluid and diaphgram get into a fight,diaphgram looses fight and leaks...which is where the hissing sound comes from.
Newer systems(91 and newer)have a dual diaphgram booster.Which can be used on the older XJs with SOME modification.....
 
okay here is the low down on the hiss

After waiting overnight brake is hard (no boost) and no hissing

start car up and apply the brake about 3/4th the way to the floor .....HISSSSSSSSSSSSSSSSSSSSSSSS... continue down to the stop and the hissing stops


turn car off wait 30 seconds..

apply brake... one good peddle push before exhuasted and no boost
 
thats the road i didnt wnat to go down but was afraid was necessary... i think im gonna up to the dual diaphram... just dread doing the plumbing work
 
well i dont know about you guys, but i have replaced my booster, just not my master cylinder and it has always made that sound. im not worried about it, and unless its affecting your braking, then i dont think you should either.
 
I was looking for a reason to upgrade anyway :)
Now I can justify it :)
My pedal acts much the same, turn off the engine, I get one and a half pedal pushes before the pedal is hard as a rock and the brakes don't work that well.
 
If the hissing stops when the pedal is all the way down with the engine running then the booster is probably okay. If, on the other hand, the hissing is always there when the pedal is held down then you're leaking vacuum at the booster.


K
 
it does stop with the pedal to the floor. Oh well, I'm probably going to redo the whole brake system soon. Some lines need replaced, I wanna ditch the rear propo valve under the bed. So I'll probably swap in the 96+ booster, an xj propo valve, and new lines all the way round. Quite a bit of work, but hey, can't skimp on the brakes.
 
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