I bet the problem's in your master cylinder.
Bench bleeding is important because there's a dead space on the forward (headlight) side that points up and will get an air bubble that won't come out when it's installed. You may have done it right, you may have done it wrong, but you're not going to have a good feel for how you did until you assemble everything else.
If it's stubborn (and it is), I'd get a dishpan and a gallon of DOT-3 and dunk the whole thing and bench bleed it that way.
Or just get another MC. They're $35 on Rockauto. If you just bought this truck with spongy brakes, not knowing its maintenance history, what would you do? Sweet, so do it again.
When you bleed it, do the pump, stomp and hold method with a helper. It shakes more bubbles loose from random dead ends in the line. Pump the pedal 5x quickly and aggressively, hold it, have the helper crack a bleeder then close it after 1/2 second before your pedal sinks more than an inch or two. Repeat a few times on each corner.