Brake Help!!!!!

I'm working on a 91 cherokee that was brought to me with no working brakes. The pedal just went strait to the floor and would only stop the truck if it was going very slow. I have bled the lines but didn't help. I also swapped out the master cylinder but also didn't help. Then i thought, maybe the proportion valve was bad or clogged. I swaped it out and still no help. There are no leaks anywhere that i can see and really don't think there is any air still in the lines.

When the vehichle isn't running the brakes feel just ok, but once you fire it up the pedal goes to the floor. So I unhooked the vac line from the intake to the booster and its just like as if the motor isn't running, ok but not great.

anyone know what is going on? is the booster probably bad? I've just never heard of them going out before, but i guess its possible.

Brake boosters go out occasionally. Definitely sounds like this one is sucking air.

everyone else is telling me that when a booster goes out the pedal gets real stiff.

everyone else is telling me that when a booster goes out the pedal gets real stiff.

I had one go out in my old 88 XJ and I had no pedal. The diaphram was busted and the pedal went to the floor.

id try bleeding the master. the shoes may need to be tightened?

i've bled the mc twice and all 4 corners twice too. I'm going to try another booster tomorow.

any other ideas?

If it was a horse i would have shot it by now...

make sure the rear shoes are adjusted right and in my experiance when the booster goes out you loose vacum assist which makes the brake pedal hard

Bringing this post back in the hopes that 99ncxj may have found a solution.

It appears that I'm experiencing the same thing. The bleeding went fine and the pedal feels normal when the truck is off. Fire it up, and the pedal loses all resistance and the braking is very minimal. I've got 4wheel disks now so there's no real adjustment in the system as in drums. I'm stumped and am looking for ideas before I start yanking the booster out.

:mad:

check to make sure all of the pistons in the calipers are moving

I had one seized caliper in the front and a frozen wheel cylinder in the rear that caused the exact problem on mine.

Jeff

Worth a shot. :thumbsup:

Of course check for leaks,,, I guess you could do a quick check on the check valve too....

Thats a good point disk brakes use a check valve usualy located in master at the line fitting. Pete M when you converted to 4disc did you swap master cyl? Although the original poster said he put in a new master and still had same problem.Hmmm....

The master, booster and front combination valve (the thing that looks like a prop valve) are all original 1988 parts as far as I know. Where is this check valve? I'm going to need a bit more direction on that.

The master, booster and front combination valve (the thing that looks like a prop valve) are all original 1988 parts as far as I know. Where is this check valve? I'm going to need a bit more direction on that.

The check valve is that lil plastic 90* fitting that hooks into the booster. Its at the end of that big diameter hose that comes off the intake. Its the size of a half-dollar around 1/2 inch thick. It will only flow air one way toward the engine away/out from the booster.

Oh, that thing. I'll go check that out as soon as the rain stops (maybe by monday?).

Oh, that thing. I'll go check that out as soon as the rain stops (maybe by monday?).

Yelp! :cheers: That valve keeps vacuum in your booster and helps it on idle.