Probably a stuck piston in the master cylinder. When's the last time you flushed your brake fluid? You should be doing it once a year or more if you play in the water with your truck. Air isn't the only thing that can get into your brake lines. Brake fluid readily absorbs water vapor right out of the air. This can cause your master cylinder parts to corrode and possible allow the piston to get stuck in the bore, which is what I believe may have happened here.

I wouldn't bother trying to fix a master cylinder. Just get a new one, bench bleed it very well and flush the rest of the old fluid from the system.