Brake Question

We are working on a 96XJ. We replaced the master cylinder thinking this was the problem. We get pressure on the brake pedal when pumping the pedal when bleeding the system, and we have run the fluid all the way through the system. multiple times!
If I leave the brake pedal alone for 5 seconds the pressure is gone and will eventually build back up after a dozen or so pump.
With the engine running we get 0 pressure. Disconnect the booster and pressure will return, but again the pedal will fade after about 5 seconds.

Is this a bad booster, or did we possibly buy a bad MC? any help is appreciated.

If you are sure you bled the MC and the rest of the system correctly... well, bad parts DO happen. Return it and try the next one.
I've even heard of entire bad batches of parts getting out there, like 5 bad MCs in a row...

Boosters go bad and the pedal gets hard, not soft.

Does it have abs?

It had abs, but this is our race jeep and they have been gone a long time.

Brendan call me tomorrow if you're still having problems and I'll come over to help.

if you can build pressure but it goes away 99% of the time there is still air trapped in the lines somewhere. Are you having the same problem as before you replaced the master cylinder?

Dumb question, but has fixed it half a dozen times:
If drum brakes, did you adjust the rears? Will give the same sensation as air in the lines but be due to lack of adjustment. Tighten up the pressure on the rears and see if that helps any :dunno:

HAD THE SAME PROBLEM WITH FULL SIZE CHEVY PO DID NOT DO THE DRUM BRAKES RITE AND IT WAS DOIN THE SAME THING

GrimmJeeper said:

Brendan call me tomorrow if you're still having problems and I'll come over to help.

if you can build pressure but it goes away 99% of the time there is still air trapped in the lines somewhere. Are you having the same problem as before you replaced the master cylinder?

Click to expand...

Hey Mike, Thanks for the offer, the dyno shop took the truck today for tuning. we will probably get it back tomorrow or Thursday. I will let you know.

in4aride said:

Dumb question, but has fixed it half a dozen times:
If drum brakes, did you adjust the rears? Will give the same sensation as air in the lines but be due to lack of adjustment. Tighten up the pressure on the rears and see if that helps any :dunno:

Click to expand...

We do have drums in the back, and I guess this could be a possibility, something to check anyway.

Previous to the issue... We ran parker and hosed up the front brake lines (long story) where they had to be pinched off. We had brakes at the time with only the rear. Well not a lot of brakes but a lot of pressure on the pedal.
We brought it to our race prep guy, and he replaced the hard lines on the front end.
We just replaced the MC, bleeding the system we ran fluid throughout and went through the entire resevoir at least two times, and saw no air at any corner by the time we were done.

I hate to say it, but you've got a hell of a monkey with a bad attitude on your hands.

i would check the rear shoe adjustment as stated above, especially since you were running with only rear brakes.

Also, there is a safety valve in the proportioning valve that trips if you lose either front or rear brakes and seals off that half of the brake system in case you have a problem so you'll still be able to stop... as far as i know this is a one time use valve and is not service-able. you may need to replace the prop valve in order to allow the fluid to flow correctly to all four wheels.

You may also have air trapped in the lines somewhere even though you've bled them numerous times, if air is trapped in the master cylinder it can be EXTREMELY difficult to clear it by bleeding at the wheels, you may need to do the bleeding procedure at the master itself by cracking the lines right where they go into it.

I'm free to help out but I'm going to be out of town for the holiday this weekend, maybe next monday we can meet up if you aren't working? or perhaps monday night.

Good luck Brendan, I hate brakes for this exact reason :rof:

Thanks Mike, will let you know come Monday. I am taking a long weekend too Thursday to Wednesday - I have been traveling so much I need some time just to hang with the kids before they go back to school.