Front brakes locked up

Dandy observation. Try to get this to act up again, then loosen the mounting nuts for the master cylinder, then pull it forward a little. If that does not let the brakes release, it is likely you installed a used master cylinder that was contaminated with a petroleum product. That would cause the rubber lip seals to grow past the fluid return ports and block them.

If the brakes do release, there's two things to look for. The first is if the push rod coming out of the power booster is adjustable. This is much more common on import vehicles, but check for that anyway. If it is adjustable, shorten it by 1/8".

Next, look for anything that is holding the brake pedal from returning the last inch or two. If your truck does not have cruise control, it uses an adjustable brake light switch. That is not self-adjusting as it is on other truck brands. Check if the mounting bracket for the switch has been bent, or if the switch was recently replaced and not adjusted correctly. That will hold the brake pedal down a little and prevent the trapped brake fluid from returning as it heats up and expands.

Do the brakes release when you loosen the master cylinder's mounting nuts? If they don't, then you still have to loosen the line nuts to get them released, the new master cylinder has to be defective or you have one made for front drum brakes. Those will have a residual check valve in the port. Those are always used with rear drum brakes to maintain ten pounds of pressure in the hydraulic system. They're used with front drum brakes too, but must never be used with disc brakes.

On a lot of master cylinders you can remove the steel line, then run in a self-tapping screw to hook the residual check valve and pull it out. Be aware though, that might affect the line's sealing surface, and there are often other differences. For example, a lot more brake fluid has to be moved to apply a wheel cylinder compared to a caliper, so the piston sizes inside the master cylinder might be different between the disc and drum applications.

Both applications look identical, and you can't tell them apart. If the old one was for front drum brakes and you have front disc brakes, the residual check valve had been removed or it had stopped holding pressure.

I never pulled a residual check valve out myself, but I've been taught about them in multiple classes. I heard of them being pulled out so the master cylinder would work with disc brakes, but I don't know if they are what the steel line butts up to, to seal. Logic would dictate if that were the case, something else would have to be placed in there for proper sealing, as that is not done by the threads on the nut. The flare, or bell, on the end of the line is where the sealing is done.