ANOTHER brake question....

Brakes seem to be a popular subject with XJ's....

I have a '94 XJ with Dana35 and ABS. I'm pretty dilligent about keeping my rear shoes adjusted, since I'm on 31's and pull trailers on occasion. My problem is this:

I don't believe my rear brakes are getting full pressure to them. The reason I say this (besides having to replace front pads at least 3 times as often as rears) is that today I had the rear end jacked up under the differential while I adjusted my shoes again. This time, just for the hell of it, I hopped in the driver's seat, started the Jeep and put it in drive (AW4), and let the rear tires spin at idle. When I stepped on the brakes, no matter how hard I pushed, the rear wheels would only slow down slightly, but not stop. Pressure was obviously getting to the rear, but something was preventing the pressure from raising any higher than a certain point.

At first I thought it had to do with the ABS, since the front wheels were stationary, and the rears were spinning. I thought the computer was limiting the pressure to the rears, but my ABS pump was not cycling, and the pedal was not pulsing.

I'm now wondering if it's the combination valve? I recall reading a few threads regarding rebuilding the combination valve, but I don't remember what the exact topic was.

Anyone have any ideas? Thanks...

Andy

I'll ask a dumb question here: did you try to stop the rear wheels with the parking brake?

That would at least confirm the shoes are adjusted correctly, and would then point to some hydraulic malady......

AndyS said:

I'm now wondering if it's the combination valve?

Click to expand...

One word: "Yes"

AZ Jeff: That's not a dumb question; I didn't use my e-brake to try to stop them. However, I DO adjust my shoes so that I have about an inch of free play in the e-brake lever before the shoes engage the drums, so I doubt adjustment is the issue.

Eagle: Well, that sounds pretty definitive. What causes this problem, and how do I repair it? (or can you point me to the thread that dealt with this issue?) Or do I just replace the combo valve with a new one?

Thanks again, guys.

If you want to be strictly squeaky-clean legal, replace it with a new one.

If you want your brakes to work and never have the problem again, rip it out and replace it with an adjustable proportioning valve from Summit Racing, Wilwood, or Mopar Performance.

OOPS! Time out! You said you have ABS, so the generic proportioning valve is not an option. I'll leave it in the post for others who may not have ABS to see, but your choices come down to replace the combo valve, or forget about having rear brakes.

There may be a way to rebuild that thing, but if there is I don't know it. Haven't taken one apart to see what makes it tick -- I'll have to grab a couple next time I hit the junk yard, just to play with.

Squeaky-clean legal, I ain't! However, squeaky-clean safe and functional, I am.

I guess I can do one of three things:

1) Tear out the ABS system that I'm constantly cursing during the winter, and use an adjustable proportioning valve.

2) Remove my defective combo valve, disassemble it if possible, and clean/repair it. This would be done with the assumption that I'd just have to replace it...

3) Continue to drive with no rear brakes and warp more rotors and eat more pads....not too appealing or safe sounding.

If I simply replace/repair the combo valve, are there any special instructions to bleeding an ABS system I should know about?

Or, how easy is it to remove an ABS system? If I were to go this route, it'd have to be able to be done in one day, plus I'd like to physically remove the ABS pump and dist.block, as well as the ABS sensors-n-stuff. What would I need to do as far as the electrical portion of this job to make it a nice clean removal?

Thanks,
Andy

Bleeding an ABS system is done the same as a normal system, PROVIDED you have opened the system in a location beyond the ABS pump/accumulator assembly.

If you introduced air into the system at the proportioning valve or farther on downstream towards the wheels, just bleed as normal. If you need to open the system near the M/C, or by the pump/accumulator, then you need the FSM to determine the proper bleeding procedure.

If the proportioning valve has gone bad, I would guess it's because of crud in the valve proper. Diassembly and cleaning might fix it (plus replacing ALL the fluid in the system so as to avoid recontaminating the valve after cleaning).

If you choose to convert your brakes to non-ABS, start by comparing the master cylinder and prop. valve from a non-ABS XJ from the same year, as that will tell you to some extent how much you will need to replace.

YOu can assume you wheel cylinders can be reused, but it's quite likely that EVERYTHING else will need to be replaced.

I had the exact same problem as you. I changed the master cylinder, combo valve and booster to one from a wrecked 1999 XJ. Stopping power improved a little but still unsatisfatory. Later I removed the rubber (piston) seal from the combo valve and now the brakes are better than it had been for ages. I do not know if the rear wheels will lock up in hard braking. I have no ABS.