Soft Brake Pedal

[Knothead]

Current project is using a 1979 K10 frame and suspension. Utilizing brake system from same, everything is new except hard lines. Drums , Rotors, proportioning valve , brake booster , Master Cylinder also were replaced. I can get a firm pedal but after cranking engine pedal fades to the floor? The brakes will hold the truck when in gear. Also will stop truck . Can’t get a good pedal with it running? Ordered another MC just to eliminate that as maybe the problem. I’m scratching my head on this one , looking for some suggestions and input. If it matters it is an 6.0 LS engine,thanks