2002 Tahoe brakes spongy after Master cyl replacement

I have replaced the master cylinder, bench bled and bled the brakes enough to replace all the fluid and remove any air in the system. When the motor is off the brakes are firm and cannot be pushed to the floor. When the motor is started the brakes can be pushed to the floor. There is no sign of leaking fluid and I possibly replaced a good master cylinder since the problem has not gone away. Any suggestions would be greatly appreciated.