My in laws 1990 2wd 4cyl. pick up has a brake failure.
Bled the brakes, all components (pads, rotors etc. are good). Pedal pumps up pressure when not runnig, but when you start the truck, the pedal fades to the floor.
Would this be an internal leak in the master cylinder?

Dear Red. Your brake booster is bad.

could be air still trapped in the system, or a bad master cylinder seals.....