brake pedal goes soft when engine is running

[guyryan100]

I'm certain there's no air in my brake lines as I've bled, pressure-bled, reverse pressure bled, and bled again. The pedal feels firm when the engine is off (I think it could be a bit firmer but there's good resistance from the pedal). When the engine is running the pedal drops to the floor with the slightest press and I have to cram it down to stop the rear tires when idling in reverse (or drive). Rear tires are off the garage floor. My cylinders are new, lines are new, rubber hoses are about 1.5 years old but have only been driven with once.