Locked up front brakes

You're already a few steps into the diagnosis. I hope I'm wrong, but what it sounds like is the brake fluid has been contaminated with a petroleum product. We know when you have an adjustable push rod in the booster, it didn't magically readjust itself to become too long, but the steps you took can be looked at as valid clues.

The first thing is to check that the brake light switch isn't misadjusted in some way that it is holding the brake pedal down a little. Next, if I understand correctly, the trapped brake fluid pressure releases if you open a bleeder screw or anything including the steel line coming out of the master cylinder. That proves the fluid is being trapped by the master cylinder and not the more common rubber flex hoses down by the calipers. The next step, when the brakes lock up again, is to loosen the two nuts holding the master cylinder to the booster, and let the master cylinder slide forward about 1/8" to 1/4". That equates to pushing the brake pedal about two inches, and is more than enough to let the pistons return fully if they were being held down partially due to that misadjusted brake light switch, or after someone misadjusted the booster's push rod. If the brakes still don't release, the only thing left is the primary rubber lip seals inside the master cylinder have grown past the fluid return ports and are blocking them. At the same time, if you remove one of the caps on the master cylinder, you'll find the rubber bladder seal is grown and mushy, and you won't be able to pop it back into the cap. That only happens with petroleum product contamination. Most commonly someone mistakenly added power steering fluid or engine oil. I've been involved with three that had transmission fluid in the brake fluid. Other contaminants include penetrating oil and gear lube. Years ago when it was standard practice to repack tapered front wheel bearings as part of a brake job, at the end of the service people would wipe the grease off their hands with a shop rag, then use a finger to pop the bladder seal back into the reservoir cap, then fill the brake fluid. Simply pushing that rubber seal back into the cap was enough for the grease residue on our fingers to contaminate the brake fluid. Today, when professionals who still rebuild calipers and wheel cylinders put those parts together, we even wash our hands first with soap and water to prevent getting fingerprint grease on the rubber parts.

I also shudder when I see someone use penetrating oil on hydraulic fittings and bleeder screws to get them loose. That oil is designed to seep into tight places, and can easily get into the brake fluid when the parts are separated.

Unfortunately the only proper repair for contaminated brake fluid is to remove all the parts that have rubber parts inside, flush and dry the steel lines, then install all new parts and bleed the system. That includes all the wheel cylinders, calipers, rubber flex hoses, master cylinder and reservoir. Also, the most-often-overlooked parts are the combination valve and the height-sensing proportioning valve. If the truck has anti-lock brakes, the hydraulic controller must also be replaced. The combination valve is on the left frame rail right under the master cylinder. The two steel lines connect those two parts. The valve assembly has rubber o-rings inside it. Most trucks and minivans use height-sensing proportioning valves because they can have such a variety of loading, front-to-rear. That valve is by the rear axle with a linkage connecting it to the axle. It maintains a balanced braking system as more weight is added to the rear. That has rubber o-rings in it too. If any part is overlooked and not replaced, the contamination will leach out of it into the new, fresh brake fluid, and it will recontaminate the entire system. This can be a very expensive repair, especially if you have to pay to have the work done.

Here's links to some articles that may be helpful:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

This second article refers to simply replacing old, moisture-laden brake fluid with new fluid. The flushing I'm referring to when the fluid is contaminated refers to washing the lines out with Brake Parts Cleaner in a spray can, then drying then with compressed air.

You might want to check out these videos too:

https://youtu.be/WdxvEQrMkBg

https://youtu.be/w7gUsj2us0U

Are you saying there was an adjustment screw as part of the brake light switch? That doesn't sound right. The plunger is like a plastic nail. It goes into a plastic tube that moves the switch contacts. These are adjusted by pulling back on the brake pedal real hard by hand, then the nail slides into the tube and it's adjusted. I've never seen one get out-of-adjustment after that because that nail is rather hard to slide. Am I describing the wrong thing?