I know this has been talked about a lot in the past. I have read all through the sticky and have not found all the answers. Here is the scenario.

1993 Dodge Ram 250 5.9 cummins rwal. Loaned to brother for several months. Got the truck back and brakes were shot (Pedal to the floor shot) and ABS and red brake light on. I did a Complete brake service on front and rear. Brake pedal is good and everything seems to be working great except the lights wont go off. I have attempted to pull the codes by grounding the single black wire connector behind the glove box for a few seconds but nothing happens. I also noticed when I mash on the brakes hard the left rear tends to lock up and skid. I have tapped on proportion valve, and combination valve. I did realize when I did the brake job and replaced some wheel cylinders I never bled the rear anti lock valve. Could this be the problem and make the lights come on? There is no mushyness or problem with the brake pedal. I figured out when I unplug the single wire on the pressure switch on the proprtion valve next to the master cylinder both the ABS and brake light goes off. What does this mean if anything? Please post your comments. Thanks.

 

Pull the wire off the combination valve. (under the master cylinder on the frame) If the lights then go out, you'll have to recenter the valve.

The other common problem is the low vacuum switch. Again you can check it by simply disconnecting it and seeing what happens.

 

okay how do i recenter the combination valve. and i dont have anything below the master cylinder inside the frame. I have a valve mounted to the booster directly below and off to the side of the master cylinder. Is this what the combination valve is?

 

Both front and rear hydraulic circuits go through the combination valve. It's function is to limit flow to a broken (low pressure) circuit and supply full pressure to the other. There's a switch on the piston that accomplishes that whose purpose is to light the brake warning light when this happens. The ABS controller also grabs that signal and shuts down the ABS.

One way to reset it that usually (but not always) works is to back off the pads or shoes on brakes that worked before the fix (The previous good circuit), then press the pedal till the light goes out. That re-centers the piston. Shoes are loosened with the adjusters. Pads by prying them away from the disk.

The combination valve is supposed to re-center automatically, but sometimes it sticks.

 

okay thank you i will give that a try next time i get a chance.

 

You can also accomplish this by opening a bleeder on the circuit which worked before and giving the pedal a brisk push. If the engine is off you can sometimes hear a small clunk when the valve shuttles.

The light will likely still be on because the valve will have moved to the other position but you then close the bleeder and apply the brakes and it should move to the neutral position.