My brakes keep locking up and sticking?

That really sounds like brake contamination. You don't have to actually see any liquid. Even very small amounts will embed into the shoes along with brake dust and ruin the shoes and the drum surface. If the shoes look dark in color at all, then they are likely contaminated. Look at the backing plate carefully for any apparent dark spots to indicate oil residue from either the brake fluid in the wheel cylinders or gear oil from the differential seals.

You had the right idea by opening the bleeder screw, but now when it acts up again, loosen the steel line at the master cylinder. If the trapped brake fluid releases from there, you either have a brake light switch that's misadjusted and holding the brake pedal down a little, or the brake fluid has been contaminated with a petroleum product.

That is the best thing I've heard all week. That tells us the brake fluid is not contaminated causing the lip seals to grow past the fluid return ports.

Next step is to open the system at various places to see where the trapped fluid will release and where it won't. A lot of trucks and minivans have height-sensing proportioning valves, but I don't see one listed for yours. If you have anti-lock brakes, look for the line coming out of the hydraulic controller that goes to the rear brakes and loosen it there. If you don't have anti-lock brakes, you'll have a combination valve on the frame right under the master cylinder. The next place would be where the steel line connects to the rubber flex hose over the rear differential.

By the way, the anti-lock brakes would be for just the rear wheels. That hydraulic controller design gives very little trouble but it's possible for a valve to corrode apart and block the fluid return passage.

If you don't have rear-wheel-anti-lock brakes, follow the two steel lines from the master cylinder down to the combination valve on the frame rail. It will have one wire for the pressure-differential valve inside the assembly. There will be three additional steel lines, one for each front wheel and one for the rear pair. Loosen the one at the rear to see if the brake fluid releases.

This is just one long circuit for the brake fluid to flow, and somewhere along it is something that's blocking its return. When you open it someplace and the fluid and brake releases, you know everything from that point back is okay. When the fluid doesn't release, you still have something in it that has the blockage in it. Just keep opening the system at various places until you narrow down the location of the restriction.

That wire wont have any voltage until the ignition switch is on. It just goes to the pressure differential switch. That turns the red brake warning light on when there's a leak in one of the two hydraulic systems. That doesn't have anything to do with the rear brake not releasing.

It would appear you solved the problem. I've never actually run into that, but given all the stuff that's in that assembly, it's understandable that it could cause that problem.

You aren't going to find this valve at the dealership, partly because if its age, and partly because there's too many variations. Because they DO have such a low failure rate, you'll have a real good chance of finding a good one in a salvage yard. Keep in mind though that the proportioning valve, which sends brake fluid to the rear and limits fluid pressure to reduce rear-wheel lockup, was carefully-calibrated to your specific truck's weight distribution and weight transfer during braking. If you can find a part number on your valve, and you find one in a salvage yard with the same number, you're home free. If there's no number on them, look for one on a truck that is the same as yours. That means the same year, same engine family, same wheel base, and same options. That valve will be different between a truck with air conditioning and one without because of the difference in weight on the front. A truck with a V-8 engine will have a different valve than one with a V-6. A dually rear axle or extended cab will be different too.

Sorry I overlooked your first reply. The combination valve can be taken apart but I would only do that to learn how they work or how they're built. We don't normally fix them so I don't know if there are critical adjustments or seals that could leak. If you do pull it apart, it would be interesting to know if you find anything obvious. Also, given that they do have such a low failure rate, it doesn't pay to go through all the work of removing it, then to put it back on and have it do the same thing.