Brake pedal goes to floor after starting

Bled all the lines at the master cylinder and wheels. (ABS system) Brake pedal is firm when engine is off, although I feel it travels a little more than it should. If the pedal is held in the depressed position and the engine started the pedal goes to the floor. Pumping does not change this.

Does this mean the ABS has trapped air in it? Can't find any air elsewhere.

Did you change the wheel cylinder or the front calipers? If rear, perhaps the back brake shoes are out of adjustment. If not, yes you may have air. GM recoomends power bleeding, but can be done manually. Start with right rear/left rear/right front/left front bleeding. This is imprortant as a brakes use a cross circuit. kepp the master cylinder full, then cycle the key on without starting the engine, until the orange ABS light goes out. Do this 10 times, as it cycles all the valves, servos etc everytime you do this, then rebleed. there are also 2 bleed valves on the ABS. Repeat as nexessary, and if it was like mine, there will be several repeats
Good luck
Jeff