Brake Booster Hiss condional on pedal postion

My brake booster leaks.. but only at about 80% pedal pressure (80% of how hard I can push) At idle, in park, I can push the pedal to the spot where it hisses, and I can hear my idle go up. If I kill the jeep, and pull the vacuum connector out of the booster there is still vacuum pulling on it. I took the master cylinder off, and pulled the rod out and the seal.. there was still plenty of grease it on from the factory, I cleaned both and greased the mess out of both. It didn't make any difference, I'm guessing that the leak must be in the actual diaphragm maybe a small tear and it seals it's self in a fold or something most of the time...

A reman one is $79... the one on my parts jeep is free.. so I'll probably go that route its just a toss up how long it will last.

I read that several times and couldn't see a question in your post.

My front tires are getting kind of low on tread, I'll probably replace them some day.

JJacobs said:

I read that several times and couldn't see a question in your post.

My front tires are getting kind of low on tread, I'll probably replace them some day.

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LOL that's a good point.. I guess what i'm asking... is if anyone knows where the leak may be? Old school mechanics used to take these apart and repair them..

My 94 does the same thing. Figure if it totally dies, I will still have braking (albeit non vac assist) and will deal with it then. Just plan ahead, and only use 79% braking or less. ;P

Sc00ter said:

My 94 does the same thing. Figure if it totally dies, I will still have braking (albeit non vac assist) and will deal with it then. Just plan ahead, and only use 79% braking or less. ;P

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hah ok, I guess I'll pull the one off my parts jeep eventually.. maybe there is also a seal where the arm is that connects to the pedal? May be that's where the leak is.

Had the same hiss on my '87. Drove it for years like that with no discernible problem other than a hiss.

Steve
Colorado

Mine is an '87 as well.. it would be my luck the one on my '89 parts jeep does the same thing.

get a master cylinder and brake booster out of a 95/96 or a grand cherokee, they're way stronger anyways and not as old so they aren't as likely to be blown. Search for "booster swap inurl:naxja.org" on Google and look under "more results", a few good threads are in there.

Just replaced my MC a couple week ago.. with a reman one.. I guess the bigger booster is not compatible with the stock MC?