Very soft brake pedal

1998 GMC k1500 v8 5.7 extended cab brakes. With engine off pressing of brakes feel hard, when you start the truck they go to the floor. Master cylinder, both rear cylinders/drum pads have been replaced. All wheels have been bled. What does the pressure distribution box do for brake lines. It has multiple lines going in. Brake pedal feel real mushy and is very close to the floor. Brakes do stop truck. I heard there was a balancing pressure device in some other trucks could this be causing my issues?