Brakes have zero resistance.

I'm having a bit of a problem, for a while now the brakes on my XJ have been really bad. I'm talking ZERO pedal feel, when the brakes start to grab (which they actually stop the vehicle), the pedal is almost at the floor.

Bleeding them helped a little bit, but nothing significant. So what I did was put all new wheel cylinders in the back today (they were really bad) and flush the system with all new fluid. The backs bleed better now, but my brakes are even more touchy, locking up in the rear when I try to stop.

So what could be the problem? It's not the booster I don't think, it's holding vaccuum quite well, it's not the wheel cylinders or air in the lines, so that leaves my Proportioning valve or the Master Cylinder. What could it be?

What do you mean "really bad" when refering to the backs? Do you have "feel" back in the pedal? Is the pedal firm now, or does it still go to the floor? Did you bleed all 4 or just the back? What year? How many miles? ABS?

Rev

It's an 1989 Cherokee Laredo with no ABS and 204,000 miles. The problem is that there isn't any feel in the pedal, it's not firm at all, the brake goes almost to the floor without any resistance, then it starts to grip, then locks up. But the difference in pressure between light braking and locking the rears is REALLY tiny, you have to be really careful when braking because just a bit too much pressure and the neighbors start to give me funny looks.

The new wheel cylinders just made it a lot easier to bleed all the brakes, which I did bleed all of them. I also cleaned out the master cylinder, it was full of sludgy crap, as were the back cylinders, so now my fluid is nice and clean. But other than that, they aren't any better than before when I still had no pedal feel.

RyleyF said:

I also cleaned out the master cylinder, it was full of sludgy crap, as were the back cylinders, .

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Where do you s'pose the crap came from? REPLACE the master...now. Brakes are nothing to mess with...ever.

JMHO

Rev

mite be having blowby in the seals in the master cylinder, check around the outside of it the engine area and under the dash . I've had some to leak to the inside on the rod that goes though the firewall. Just a easy quick check idea to look at

Rev Den said:

Where do you s'pose the crap came from? REPLACE the master...now. Brakes are nothing to mess with...ever.

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I just figured the crap was build up from 16 years of existence. And you can rest easy, I'm not actually driving this thing on open roads or anything like that, I'm just keeping it under 20 around the neighborhood trying to get it fixed.

RyleyF said:

I just figured the crap was build up from 16 years of existence.

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Most likely.

However, you really should replace the master cylinder (this is one of those things that is *not* worth rebuilding from a safety standpoint). Chances are good that its seals are shot, leading to blowby as JMH suggested. While you're at it, doing the booster probably wouldn't hurt either - another possibility is that the diaphragm and/or valve inside of it is knackered. Coupled with a shot MC seal, your fluid could be ending up in there.

have you checked the front brakes at all? i agree with replacing the MC and booster too.. that "sludge" could very well be your internal seals broken-down over the years..

Sounds like the combination valve (that junction block just below the master cylinder) may be clogged up. There is a sliding piston and o-ring inside that, and if the rear wheel cylinders were like suoer glue ... well, the combo valve is the proportioning valve for the rear circuit, so it's probably super glued inside as well. The FSM says it cannot be serviced, only replaced. Don't know how much a new one costs, but my brakes have become progressively worse and I'm probably going to replace that with my next brake job.