Brake Diagnosis..

[Xhavier_5478]

I am in need of some advice or onsite expertise, my cherokee has the Crown Victoria rear disc swap on a Dana 44, and the WJ big brake swap up front. I set it all up, added ZJ spring and internals to my prop valve, bled the system, and things were great. Since this is a jeep I've been working on about 5 different projects on it at anyone time, well it sat for a few weeks while I worked on other issues, when I went to drive it the next time the brakes were dragging, so I pulled the rear wheels and both calipers were extended, so I pressed them back in with a c-clamp and did the same in the front, drove fine after that. Let it sit for a week or two again this time only the fronts, so I figured they were bad calipers from the Pull and Pay so I should replace them and get new ones, did that, bled the system and they were fine, fast forward a few months my jeep sat while I was doing my drive train swap, it did it again I forced the rear piston in and it was fine.
Last night after the jeep has sat 3 weeks, I actually had to bleed off some fluid from the rear caliper to force the piston back in, and the front wouldn't retract till I let some fluid out of it also, after that all are fine.

I'm going to check them again tonight and see if they are the same situation.

The question is why would they accumulate pressure from sitting?

Does anyone run the Crown Vic disc/WJ disc combo, and did you have to go to a different Proportioning valve all together?

 

[old_man]

Calipers don't retract. They only relieve the pressure and will float just off the rotor.

 

[Hypoid]

To follow up Tom's information, letting a vehicle sit will give the rotors time to rust. Rusty rotors feel way different from clean rotors. Next time you let the vehicle sit, have a look at the rotors before you start driving.

FWIW, if the pads are good, and the rotors are rusty, I make a point of putting pressure on the brake pedal while I am driving. It does not take much to change the way the brakes feel, one way or the other...

 

[Xhavier_5478]

old_man, Yes, you are correct, in this case the pressure is not being relieved from them.

Hypoid, Yes, the rotors had become rusty, but this was pressure to the point of when letting off the gas the vehicle would not coast, and confirmed when I jacked up the rear and then the front and tried to rotate the wheels, they were not having it.

 

[eflores]

sounds like you screwed up putting all the parts back in your prop valve. gut it and rebleed.
let it sit and see if it still does it

 

[Xhavier_5478]

So I just drove it a little over a mile and the rears are locked up, so I'm going to pop the guts out of the prop valve.

 

[Xhavier_5478]

So I pulled the guts out of the prop valve, and inserted the piston and spring from a stock 92 XJ and I was able to drive it home. I drove it around the block a few times just to see if I could get the rears to lock up again, but they wouldn't. I'm going to bleed the system tonight and see if I can recreate the situation.

 

[Xhavier_5478]

So I bled the brakes this evening, and everything was feeling good, I drove it about 2 miles, and brakes were feeling awesome. Then I let it sit for about an hour and then drove a mile in it to get some gas, when I was at the gas station I felt the rear disc, and it was too hot to touch, I got in it and drove another 2 miles, and by the time I was pulling into my driveway it was stressing to overcome the brakes..

This is with stock proportioning valve from jeep cherokee of the same year.

I'm starting to get frustrated here.

 

[eflores]

so try gutting the prop valve. perhaps the housing has issues and the plunger is binding

 

[Hypoid]

Apples to oranges, but not long ago a guy I know had a similar problem with his ZJ. After replacing the ABS pump, he found a little debris in one of the ports of the old pump. He probably put a half gallon of brake fluid in that thing trying to bleed the brakes. It is difficult to imagine having any trash left in the system, but there it was.

 

[Xhavier_5478]

It turned out to be the pushrod adjustment inside the vacuum booster.

 

[eflores]

lol who adjusted it

 

[Xhavier_5478]

It wasn't adjusted when I swapped the vacuum booster and MC, and the brake light switch was extended in the mount, pushing the pedal some also.