Brakes go to floor when motor is cranked

replaced leaking rear wheel cylinders, shoes, and drums.After bleeding until no bubbles, and firm pedal, started engine and the pedal dropped almost to the floor.Bled more.Had small leak under reservoir, so I replaced master cylinder.Bled more.Have firm pedal, till I start the engine.Bled master,bled all 4 brakes,again. So far used 3 qt,s of fluid.Can it be the ABS hydraulic unit? Pedal is firm and does not drop until I start it up.