Brakes 82 K20

[akart]

Bleeding the brakes after new rotors. Brakes will stop the truck but the peddle continues to go down. All new parts except the master/booster and the flex brake lines. These will be replaced next. With the engine off I have good peddle and doesn't go down but goes down when engine is running. And am I supposed to bleed the brakes w/the engine on or off. Brake fluid level remains constant, no apparent leaks. I'm thinking diaphragm??? Thank You