Definitely a clogged port in the master cylinder. The fluid was low because it is trapped in the lines. This is causing the pedal to be hard and the brakes to lock down. It is just like having the brakes to the floor. If it were my truck, I would take the opportunity to upgrade to a dual master cylinder and replace all lines (it sounds like they are rusted or gummed up) including the rubber hoses at each wheel. Brakes are nothing to be guessing about. I don't know which is more dangerous, no brakes or locked down brakes. Good luck

Phil

Never hold your farts in. They travel up your spine and into your brain...that is where crappy ideas come from.