usually when the brakes lock up like that it is generally caused by a improperly adjusted rod not letting the piston inside the master cylinder return completely. It does not allow the fluid to return back to the resivoir and turns the master cylinder into a pump and it keeps pumping up the system till it locks down.

i need to ask
does this truck have the vertical master cylinders mounted just under the steering column/wheel in the center under the dash? or is it a normal type mounted on the frame under the floor plate area?
If they are the vertical ones under the dash those are a real pain in the keester to get bled out and adjusted properly.