Brake Help!!!

[99NCXJ]

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.

 

[99NCXJ]

someone throw out some ideas...its getting ready to get ugly around here.

 

[redneckboarder]

are you losing fluid without seeing it on the ground? i hear the MC can leak into the booster sometimes. are your calipers on correctly, and wheel cylinders good?

 

[8Mud]

You didn't mention if it has ABS or not, the priming and bleeding sequence with ABS can be complicated.
The master cylinder often has to be primed (air removed and replaced with brake fluid). If it isn't, it can let the pedal go right to the floor.
What happens to the reservoir level when you push the brakes?
You have to ask yourself where the fluid is going? Around the piston in the master cylinder? There isn't any fluid in the right places in the master cylinder (Just air) and the fluid isn't going much of anywhere.
Or possibly the rear wheel cylinder are too big, the shoes not adjusted right, the front pads are worn and the master cylinder lacks the volume to work it all (it happens).
It's likely you have trapped air in the master cylinder, you can try cracking the lines slightly at the master cylinder and seeing if air vents. Use a big rag, avoid getting brake fluid on anything, if you do, wash it down with soap and water, rinse well.
I've had to completely submerge an occasional brake master cylinder to get it to prime correctly. This is kind of extreme, but it happens.

 

[99NCXJ]

I'm not loosing fluid. No leaks anywhere.

Its not an abs system.

I have bench bled the master cyl. twice and still not better.

with the cap off the resivoir and you push the brake pedal you can see a stream of fluid (under the surface of the fluid) streaming up.

I have gotten the brakes to feel better by doing more bleeding and replacing some hard lines on the rear but they only feel better when the motor isn't running.

once you crank it up the pedal will just go to the floor, unless you disconnect the vac line going from the intake to the booster, then it acts good.

 

[poorboy_616]

Bad booster will cause the no brakes under vacuum assist....

Also, REBLEED ALL FOUR CORNERS!!

When I replaced the master on my 87 three months ago, I had to bleed it three times all the way around the get good pedal....

 

[99NCXJ]

put another booster on and it got much better. stil not perfect but atleast i've got something to work with now. going to re bleed for the 5th time and keep my fingers crossed.

 

[jeeptx23]

post what you find

 

[8Mud]

Pull the rubber boots off of the rear brake cylinders, carefully so they don't tear, see if there isn't fluid in there on the wrong side of the piston.
I still say the fluid is going somewhere, the booster just makes the pedal easier to push, it really has nothing to do with the fluid. Unless the fluid is leaking into the booster past the seals.
Just for the heck of it, pull the parking brake up all the way and try again.

 

[99NCXJ]

no fluid leaking, unless its leaking past the piston in the master cyl. and staying in the mc.

The rear brake cylinders are in perfect shape with fluid where there should be.

I bled them again...really getting tired of this thing. I'm going to get a vacuum pump and try bleeding them that way tomorow.

 

[rweaver138]

haha stubborn old jeeps. i had a 90 xj that, no matter what i did, couldnt get the pedal to come up. replaced everything BUT the booster. i just said **** it and dealt with the bad brakes.

 

[in2fords]

Im doing the same thing on mine right now. I've replaced everything w/ new peices including the lines/hose's but the master and booster but my pedal still goes straight to the floor My buddy has a 2004 tj so I will see if the mc/booster from that will fit. If not Im just buying new stock ones and getting it done.
If my rig isnt ready to go on the boat by the end of this month I will need to leave it behind and come back in a few months

 

[prb24]

Might sound stupid...but check to see if the last guy put the calipers on the wrong sides. That will put the bleeders on the bottom, and you'll never get the air out.

Don't ask me how I know...

 

[8Mud]

Bleeding the master cylinder can be a pain, about one in ten I've replaced needed some serious bleeding to get them to work right.
There are actually two segregated systems (front and rear) in one, one piston pushes against the other, the work in tandem. Air between the pistons can be hard to move, especially from up top to the bottom bleeders. Sometimes before you can get the lines reconnected to the master cylinder, you loose part or all of your prime. I have plastic plugs to keep the prime, when they aren't handy I use a small piece of paper toweL (paper towel doesn't fall apart or shred easily, sucks up the fluid and makes a fair seal).
The chances of both pistons passing fluid around the seals is unlikely, most likely it's air.
The master cylinder is designed to be failsafe, front half may go out, the back half may go out, but the chances of both front and back failing at once, should be fairly remote.

I once got a replacement master cylinder, that was assembled wrong. Had an old single stage master cylinder, that failed (rusty bore). The replacement refused to prime. I found the spring installed on the wrong end of the piston in the new one (made in Mexico).
Had another that had so much trash in the brake lines, that chunks blocked the orifices in the master cylinder. That one also refused to prime, until I cleaned out the master cylinder, replaced all the brake fluid and flushed the lines

 

[in2fords]

Might sound stupid...but check to see if the last guy put the calipers on the wrong sides. That will put the bleeders on the bottom, and you'll never get the air out.

Don't ask me how I know...

I never thought of it, just went out and looked and I will be removing and reinstalling front calipers today along with rebleeding the master.
the sad thing is Im the guy that replaced the calipers :eyes:

 

[JXPEEJ]

This seems to be going around.

 

[in2fords]

That was it, had the calipers on wrong sides:laugh2:
sometimes I cant see the forest thru the trees:laugh:
its nice having fully working brakes, took the Jeep for its first cruise longer then my driveway today:thumbup: it feels really good to beable to drive it after wrenching on it for over a year. NOTHING feels familiar about it, course I did convert from 2wd/pungent 5 speed to a completely rebuilt 4.0/4wd/np435/205 and a 4" lift with 33"s.