2003 Ford Taurus

Brakes problem
2003 Ford Taurus 6 cyl Front Wheel Drive Automatic 90000 miles

The brake pedal started going to the floor( evidence of leakage from rear of master cylinder also), so I replaced the master cylinder( I bench bled it) The problem continued so I replaced it one more time. Same thing. When the engine is off I can pump the brakes up to a good height and pressure. As soon as the engine is stared the pedal falls to the floor. No leaks were detected around the wheels.
February 21, 2010.

Check rear wheel cylinders if leaking
adjust rear shoes
Pump and bleed the brake system with engine off

Feb 21, 2010.