Proportioning valve question...

Alright, I'm having some trouble with the brakes on my 87 XJ. At first, it started out with the rear brakes acting funky... they would grab much earlier than the fronts on occasion (usually it happened constantly for a few days, then went away and everything was fine).

Lately the brake service light (the idiot e-brake light) has been coming on under hard braking, which normally means a loss of pressure to the rear brakes. However, I've found that if the light is on, I can make it turn off by pumping the brakes once, and then pressing down firmly (I usually do this at a stop light or a stop sign or something). When I do this, I hear/feel (through the pedal) kind of a "POP" and a quick loss of pressure completely on the brakes, then they grab again a split second later, and the light goes out, and all is fine until I have to brake hard again (as in, someone steps out in front of me in a crosswalk or something, REALLY hard braking).

So, my question is, could this be a bad prop valve, or should I start looking at the rear brakes? I have the dreaded big drums on a factory D44 rear end, which are the brakes most people have problems with. I haven't inspected the rear brakes yet as it will require some force to remove the drums (they're on there pretty solid). I'm just trying to rule out some more simple stuff first... here are the things I've done so far:

Checked for leaks, none at all unless it's pooled up inside the rear drums and not seeping out

Checked fluid resevoir, no problems there it wasn't low at all

Bled front and rear brakes, no air in the lines to begin with

Disconnected E-brake to make sure it wasn't hung up or causing problems

Attempted to adjust rear brakes (not sure I got it completely right, but they did feel a little better afterwards)

Anyone have any suggestions before I jack it up and start pulling stuff apart? Does this sound like a faulty proportioning valve to anyone? From my understanding, they have a piston inside that moves side to side.. if the pressure on either side is less, it moves far enough to trip the light... so could it just be getting stuck to one side and applying enough pressure frees it up? Then again, if that was the case, there would have to be a low pressure instance on one side for it to even get over to trip the light... so i dunno. What's everybody think?

????

You have a few misconceptions to deal with before you can diagnose your problem.

First, the brake warning light goes on if EITHER brake circuit -- front or rear -- loses pressure. The fact that it lights up under hard braking certainly indicates a problem, but it doesn't automatically mean the problem is in the rear brakes.

Secondly, the larger drums on the D44 axles are not "dreaded." They provide more and better braking than the smaller D35 rear brakes. Wherever did you get the idea there's some kind of problem with them?

Diagnostic question #1 -- Have you crawled under the vehicle and looked for evidence of brake fluid, either dripping out of the rear drums or seeping out of any of the hard lines running along the frame rail?

Diagnostic question #2 -- have you removed the rear drums to visually inspect the rear cylinders for leakage/seepage?

Diagnostic question #3 -- When's the last time you adjusted the rear brakes? Yeah, I know it says they are "self-adjusting," but even when they work it requires several HARD stops in reverse to activate the self adjusters, and they usually don't work anyway. If the rear shows aren't adjusted properly, the pistons in the wheel cylinders have to move too far (too much fluid) before the brakes engage, and that could be enough to fool the combo valve into thinking the rear circuit failed.

i'm thinking the problem is closer to the master cylinder than the rear.... if you are holding pressure on the pedal and it continues to go down and gthere is no evidence of external leakage its most likely bleeding back past the master cylinder piston.. as of last year the master cylinder rebuild kit was still available from my local advance auto.. i rebuilt the cylinder in my 87 and it cured a similar "bleeding down" problem..

mike