You can't 'preload' the brakes, they'll lock up.
You'll need to re-evaluate what you think you have, if the pedal goes nearly to the floor, then, the front brakes cannot be stopping 'fairly well' - see what I mean?
I'm not trying to pick apart what you're saying, just giving you a different perspective.
What I would be doing is some serious brake bleeding, it is a pita in that truck because of where the MC is located but I'd be bleeding the MC and working my way to the 4 corners, you'll find trapped air all over the place. You'll probably have some leaks to deal with once the system gets up to pressure. Dot3 fluid.
