After changing front pads , replacing wheel cylinders in rear and the master cylinder ,why can I not bleed the system to where it should be?
If it’s an old van, and you still have a soft pedal, I’d suspect that the brake hoses (rubber) are swelling each time you push the brake pedal. You can actually observe the brake hose as someone depresses the brake pedal. If you can see it swell, change it (them). If old, change them anyway (interior deterioration).
Try cracking the brake line fittings open at the master cylinder while someone presses on the pedal. Tip the truck up a bit. The side the lines are attached to while you do it.