hissing brakes and not enough of them! (long post)

Well i've searched and searched, read over 500 posts and not found an answer to my problem! I have an 89 cherokee laredo 4.0 aw4 with 98,000 supposed miles that i've owned since last december. Recently i found a wire that was not connected to the master cylinder that i assume was the brake failure sensor. i hooked it up and bascially nothing happened. Two weeks later, I noticed a marked reduction of braking power. I thought maybe it was something to do with the wire (probably not). At any rate, it has gotten real bad. my brakes feel like they are barely there. My parking brake works just fine. Here's the thing, I feel it is a vacumn problem because when i apply the brakes i here a hissing underneath the steering coulumn. I am hoping it is a vacum line off somewhere under the dashboard, but i have a bad back and am having problems looking. i am going to have my son look there to see if he can see or here where it is coming from. i saw a post that there is a vacum line hooked to the cruise control, if this is unhooked or ruptured would this cause a hissing and lack of brakes? to go further with this, I notice that when i hold the brake pedal down (engine running) there is a hissing sound and the rpms go up!!! I'm still thinking a vacumn issue and not a full out power brake or booster loss. as it is an 89 and i don't see a abs system attached, i assume it is a conventional system. ironically, this is the month i'm due for my safety inspection so as murphy's law would suggest, whatever can go wrong, will go wrong at the worst possible time!!! The hissing is so loud under the dash that i'm hoping one of you fine people could direct me in the manner of a vaccum line coming off near the pedal. it is equipped with cruise which has not worked since i got it. The brakes were "new" when i bought it and had plenty of stopping power. The seller gave me the receipts. I just feel it would be throwing good money after bad to replace the master cylinder if it is just a ruptured vaccum line. I saw a post earlier that stated that you should pump up the brakes and start the vehicle to see that the pedal falls, it does big time but never pumps up again when the vehicle is started. You literally can't pump the brakes up when it's running, it stays the same, all spongey with huge travel. I can't even lock them up, except the front in low speed (like 5mph).
the hissing thing under the dash has got me thinking it's a broken vaccum line. any ideas? you guys solve this and i'll be sure and get my membership dues in one way or another even though i have personal finance issues related to my wife's disablilty (ridiculous medical bills, she has m.s.), actually, either way i am going to join, though i don't go off roading or have time to go to events, i feel you've saved me the amount i would have spent on dues anyway! you all "rock" and have been a great deal of help since i've bought the jeep.
thanks for any help you can give me,

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Classic symptoms of a bad vacuum booster: high pedal effort, hissing noises which occur when you push the pedal, and increase of engine speed when you hold pedal down.

Dollars to donuts you need a new booster. If the pedal is hard and does not sink under steady pressure or drop to the floor, you do not need a new master cylinder. Boosters have a long service life, so if you have a good junkyard in your area that warranties parts, you might consider a used one. But it's usually prudent to get a brand new O-ring or whatever other seal there is for the interface between booster and master cylinder. And make sure you get the right booster. Later models were improved, but you cannot retrofit them without some adaptation, so if all you need is to get it back the way it was, stick to the type you have.

Its the booster, No doubt. New or rebuilt ones can be had for about $100. I'd go that route, rather than risk a junkyard one (rubber dries out in junkyard heat, etc.), especially if you have to pay someone to install. I'd swap mster cylinder at the same time unless its new or nearly so. Add about $75 for that or so.

Pretty easy job to put in, if you know how to bench blled MC and bleed brakes. With a bad back though, you'd prolly want help hooking up to the pedal.

Nevada City Sparky said:

Its the booster, No doubt. New or rebuilt ones can be had for about $100. I'd go that route, rather than risk a junkyard one (rubber dries out in junkyard heat, etc.), especially if you have to pay someone to install. I'd swap mster cylinder at the same time unless its new or nearly so. Add about $75 for that or so.

Pretty easy job to put in, if you know how to bench blled MC and bleed brakes. With a bad back though, you'd prolly want help hooking up to the pedal.

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I'm told that the power booster does not require a brake bleeding, only the master cylinder. Do you know if this is so? I'm thinking of going one of two ways, autozone has the power booster in stock at my local location, but i have to mail order the master cylinder. If i mail order the whole deal i'm only saving like five dollars so I'm thinking of trying the power booster first, and then if this doesn't work, going with the master cylinder (it really doesn't look that old). One thing i failed to mention, the brake fluid in both sides looks like caca, i.e., it is murky brown instead of clear, so i'm thinking i have to bleed them anyway. It's strange because the brakes worked great for six months or so and then just went away.

davehoose said:

I'm told that the power booster does not require a brake bleeding, only the master cylinder. Do you know if this is so? I'm thinking of going one of two ways, autozone has the power booster in stock at my local location, but i have to mail order the master cylinder. If i mail order the whole deal i'm only saving like five dollars so I'm thinking of trying the power booster first, and then if this doesn't work, going with the master cylinder (it really doesn't look that old). One thing i failed to mention, the brake fluid in both sides looks like caca, i.e., it is murky brown instead of clear, so i'm thinking i have to bleed them anyway. It's strange because the brakes worked great for six months or so and then just went away.

Click to expand...

Yes, you can do the booster without bleeding, whereas the MC does require bleeding. However, it isn't really all that hard to do the bleed, and it sounds as if you could use a fluid flush anyway. Your choice, though. You could do the booster and still do a fluid flush by bleeding. Or you could replace the master cylinder later too. The main advantage to doing both at once is simply that it gets everything done together, no problems with seals between MC and booster, etc., and of course once done it should stay done for a long time. I'd decide on the basis of budget, potential future time spent, and who is doing the work. Do the whole job if you're paying someone to do it, because labor will be your big cost.