1979 Ford F-100 Brakes Sticking

You've handled the more common causes. How about a brake light switch that's out of adjustment and holding the pedal down a little? There's two different styles of switch and one can be misadjusted. The clue is the brakes will release if you loosen the master cylinder's mounting bolts a little.

The next time this happens, open a bleeder screw or loosen the steel lines at the master cylinder to see where the brake fluid is being trapped.

When the brakes lock up, I open a bleeder screw on the front wheel and they release for a while, then lock up again.

That tells us there's fluid getting trapped and it's not the fault of the calipers. When it does it again, loosen the steel lines at the master cylinder and see if that lets the brakes release.

If the brakes release after I loosen the lines at the master cylinder, what does that indicate? Is the master cylinder bad?

You said you replaced it already, so that leaves just two possibilities. One is something is holding the brake pedal down a little. That is almost always the brake light switch. The second is the brake fluid is contaminated with a petroleum product. The clue there is the new master cylinder would have solved the problem for a day or two but it would have occurred again. The only fix for contaminated fluid is to remove every part that has rubber in it that contacts the brake fluid and to flush and dry the steel lines, then replace the parts and put in fresh, new fluid. If any rubber part is not replaced, the contamination will leach out of it and recontaminate all the other new parts. That includes calipers, wheel cylinders, rubber flex hoses, master cylinder, combination valve, and height-sensing proportioning valve. A '79 might not have the last one.