Your drum/drum pressure differential valve should look like the one in the following photo. It will be located on the drivers side inner frame rail, just rearward of the steering gear box.
There will be (3) lines on one end and (2) lines on the other. The end with (3) lines is the primary side for the front brakes. The end with (2) lines is the secondary side for the rear brakes.
As long as there's equal pressure in the brake system, the pressure differential valve spool will remain centered. If there's a failure in the brake system (on the rear for example), where a hard line or flexible brake line ruptures or the wheel cylinder blows out, this will cause low pressure in the system to the secondary side of the valve and the higher pressure of the primary side of the valve will force the differential valve spool to shift to the lower pressure secondary side, which would turn the warning light on. This can also happen when you are bleeding the brakes out. The system doesn't know if it's a legitimate component failure or if you're just bleeding the brakes out. It just knows it senses a differential in brake pressure between the primary and secondary brake circuits, the spool moves to the low pressure side and the warning light comes on.
You will need to take the wiring connector off the plastic switch, sticking out of the valve, and unscrew the switch from the valve body to look in through the hole to see where the position of the detent (narrow part of the spool) is. The spool inside is hour glass shaped. The detent of the spool should be centered in the hole. You may need a mirror and a flashlight to see into the hole.
If you find the detent is off center, towards the secondary side (rear brake circuit) for example, then you will need to put the plastic switch back in the valve and reconnect the wiring connector to the switch. You'll have to crack a bleeder screw open on the primary side (front brakes). From inside the truck, turn the key to on and watch the warning light as you begin to slowly apply pressure to the brake pedal. This will build pressure on the secondary side of the brake system. The increasing pressure should get to a point that it forces the spool back to center. The instant the light goes off, get off the pedal or the spool will shift all the way over to the primary side and the problem will go from the rear circuit to the front circuit. This would probably be easier to do with two people.
Before you start the process of correcting the valve, this simple valve lock tool (in the following link) is a very good item to have to prevent this from happening when you are bleeding the brakes. It won't re-center the valve if it's already off-center to begin with but, it will keep it from moving in the future when you need to bleed the brakes. --It may be that during this process of re-centering it that you will have to re-bleed the brakes. If so, it will save you a lot of trouble now, AFTER you get the spool back to center.
https://www.musclecarresearch.com/brake-valve-tool
