2001 Audi A6 Brake pedal spongy and goes to floor

Brakes problem
2001 Audi A6 4 cyl Front Wheel Drive Automatic 150k miles

I have changed 2 new master cylinders bled the system, pressure bled and manual bleeding, no fault codes no abs light on, No fluid loss, brake pedal firm when engine off but when engine on and slight pressure foot resting on pedal it slowly goes to the floor, on rolling road brake effort deminishes as pedal gradually go to floor, any emergency or aggressive braking result in excessive pedal travel with very weak brake, I also tried braking on gravel and abs comes into. At the moment not been able to obtain any tech details of the hydraulic side of this system to study how it works and which valve seal might be leaking, within the abs unit itself, I would realy like to understand this problem so any reference data to able to do read up, would be great.I dont mind paying, for this but need more info on how to make donation thanks Richard