Brakes keep locking up

When it is acting up, park on a slight incline and let it sit in neutral. Prepare yourself in case the brake lets go. You'll look funny chasing it down the hill like I did once! A block of wood a few inches downhill from one tire will work.

Crack open the steel lines at the master cylinder. If the reservoir cap is loose, no air will get into the hydraulic system. If the brake lets go, suspect the brake fluid is contaminated with a petroleum product. That will be a real expensive problem that we'll have to discuss further. If that doesn't help, open the bleeder screw on the front caliper that's getting hot. If the brake releases, look for a metal anchor bracket in the middle of the rubber hose. Rust can build up inside where the bracket is crimped around the hose, constricting it. This will also cause an abnormally hard and high brake pedal. That crimp can be opened up slightly with a large pliers. When constricted this way, the high fluid pressure from the brake pedal can force the brake to apply, but the fluid can't return to the reservoir, so the caliper stays applied. As it heats up, the fluid heats up, expands, and applies the brake even harder. This can occur, but less-commonly at the metal fittings on the ends of the hose, but this won't be the cause if you replaced those hoses already.

If opening the bleeder screw does not release the brake, suspect dirt or rust pits on the caliper piston. It's not cost-effective to rebuild them like we did in the 1980s. Replacement calipers are fairly inexpensive, but for even braking, both front ones should be replaced at the same time.

Let me know what you find.

This is common when the brake booster push rod needs to be adjusted inward. When the brakes are locked please loosen the brake master mounting bolts if this is the problem the truck will roll once again. Please let us know.

This isn't a wheel bearing issue. I also doubt it's related to brake fluid contamination as the trapped fluid wouldn't release that quickly. I really suspect a rubber flex hose issue. They can tear inside forming a one-way check valve, then the fluid can only seep through it slowly to release. Rust under the crimped ends might do that too. This is more common on vehicles that have a metal bracket crimped around the middle of the hose. Simply opening that crimp up a little with a large flat-blade screwdriver solves those.

The booster's push rod is of an unusual design and it's recessed to where you can't get at it. Adjustable push rods like Ken mentioned used to be common on import vehicles, but the suggestion is still sound as for loosening the master cylinder's mounting nuts a few turns. Along with GM's uncommon designs comes some unusual problems too. Some of their boosters actually have two separate diaphragms inside, and both are needed to get the proper power assist, and either one can develop a problem that might hold the master cylinder partially-applied. This is where it will release if the mounting nuts are loosened.

Did you try to loosen the brake master mounting bolts?

Hi GBOLL24
I would second the brake booster there is a technical service bulletin( #PIT3665J: Brake Pedal Moves or Self Applies at Full Turns - (Jun 26, 2020))
Condition / Concern
On vehicles with a Hydro-Boost System, customers may notice during full lock turns or near full lock turns that the brake pedal will move down or self apply. This can be caused by the internal dump valve of the hydro-boost not seating properly due to contamination in the steering system. The hydro boost assembly is not tolerant to certain types of contaminates that may be present in the steering system.
Recommendations / Instructions
The Hydro-Boost System is not serviceable and must be replaced.
Important: †Before replacement of the Hydro-Boost System, the entire steering system (including power steering pump, lines, cooler, and gear box) must be flushed to avoid a reoccurrence.
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Check out the diagrams (below).

Cheers, Nick