proportioning valve replacement

Hi,
99 xj with abs, problem is that the rear wheels lock up before the front wheels causing the abs to always fire when reaching a low speed (no brake light or abs light). I purchased a proportioning valve and want to replace it.
When replacing the proportioning valve do i have to bleed the abs system (with a scan tool) or is a base bleed good enough?
Also what else can cause this problem (my front tires are 205 75 15 rear are 215 75 15) can that cause a problem as well?

It would be very logical to assume that having two different sizes of tires installed is the source of the ABS activation. Get 4 similar sized tires installed and the ABS should function correctly.

When working on the 1997-2001 ABS brakes, you should disconnect the battery and then custom cut a 2x4 to depress the brake pedal that is held in place by the steering wheel or seat frame Depressing the brake pedal shuts the ABS ports and prevent air from entering the ABS pump. Disconnecting the battery prevents discharging the battery with the brake lamps.

If you get air bubbles into the ABS pump the dealership will have to bleed the ABS with the DRB-III Scan Tool. Some people claim that you can trigger the ABS pump and bleed the ABS, but I don't know if that actually works.

Unless you alter your brake system (different calipers/rotors in front or a rear disc swap) there is no reason to mess with the stock prop valve. In your case the different sized tires (and compounds which cause the the rear to lock up first)are probably the cause which can also be harmful to the drive train in 4x4 in certain situations. That or your rear brakes are sticking.

Also, a prop valve is not a drop in proposition unless you a) use adapters or b) re flare the stock brake lines.

Talyn said:

Unless you alter your brake system (different calipers/rotors in front or a rear disc swap) there is no reason to mess with the stock prop valve. In your case the different sized tires (and compounds which cause the the rear to lock up first)are probably the cause which can also be harmful to the drive train in 4x4 in certain situations. That or your rear brakes are sticking.

Also, a prop valve is not a drop in proposition unless you a) use adapters or b) re flare the stock brake lines.

Click to expand...

Ok, I won't do the prop valve. I did change the shoes as well as wheel cylinders. My mechanic noticed that when the car is on the lift and he gives gas than presses the brakes the right rear wheel still spins somewhat, does that mean anything? He said since I don't have an abs light he doesn't know where to start.
Also since i took out the abs pump fuse there is no firing anymore, but I am looking for a solution, lol

Are you sure that the rear brakes are really locking up or is the ABS just activating?

Talyn said:

Are you sure that the rear brakes are really locking up or is the ABS just activating?

Click to expand...

I think abs being activated, since when i pulled the abs pump fuse i have no problem.

Fix the mismatched tire sizes first, then see if you still have problems. The size mismatch could very well be the only reason you are having any problems with the ABS. On top of that, not a good situation for a 4x4 to have mismatched tire sizes front and rear.

The simple stuff first...

MNgopher said:

Fix the mismatched tire sizes first, then see if you still have problems. The size mismatch could very well be the only reason you are having any problems with the ABS. On top of that, not a good situation for a 4x4 to have mismatched tire sizes front and rear.

The simple stuff first...

Click to expand...

Thanks, my xj only has rwd not 4x4 (although i wish it did). Can it effect something besides abs if i keep the mismatched tires on?

If your Cherokee is 2wd and you have the ABS disabled, you can keep your goofy sized tires and have no issues.

Since the rear tires' diameter and circumference is slightly larger than the fronts, the system sees the wheels as rotating slower, therefore activating the system, trying to make the fronts rotate slower, to match the rears. Unless the vehicle is set up to compensate, all tires should be identical in size, preferably the same brand and model, as there may be small differences between brands. All you're doing is fooling the system, if it's working properly. If it's working properly, and one rear wheel stops before the other, it may mean a partial blockage or adjustment problem between the rears. Fix the tire variable first.........

Another thing to check is the sensor gap. IIRC the rear is adjustable. But I may be wrong.