The brake pedal goes to the floor when running

We needed new front rotors, and pads all around. It had a full pedal when the car was not running but pedal sank to floor while applying pressure when car was running. we then assumed an internal leak in master cylinder. We replaced the master cylinder with no improvement. We were told that it was the ABS computer module, we replaced that too, no improvement. What can cause this? we bled brakes three times, no air! Can it be a sensor or what? Asked a local GM mechanic, and he has not come across this one yet! Need help! Thanks, Calfman