Replaced the master cylinder bled brakes, good pedal when not running but goes to floor when started

Yes, bled it with the engine off and had good pedal but when I started the truck my pedal instantly goes completely to the floor.

Im going on my third Master Cylinder. Autozone sells crap parts and I will not buy from them anymore. Im buying an AcDelco Master Cylinder ( That happens to be the same price as the cheap Fenco from Autozone) So under warranty im going to exchange them. Im doing this on a 98 chevy c1500 so I feel your pain. Ive had to change almost the entire brake system and im only at 148,000.

Brake booster, ABS SYSTEM!, brake lines (most of them) Two master Cylinders (Thus far), bled it several times. I still have a squishy petal but it stops. Next is rotors, and wheels bearings. Also I've changed both Calipers and wheels cylinder and turned the drums and changed shoes.

The 98 Chevy pickup truck with anti-lock brakes needs to have the system running with it engine OBD2 diagnostic system hooked up to cycle the anti-lock brakes to bleed that pump system out.