1990 bonneville brakes go soft when running

hi i have a 90 bonneville with 3.8 v6 and it has over 200,000. and i just replaced a brake line on the back and bleed the brakes. and the brakes are hard and seem to work fine when the car is not running and soon as you start the car the brakes go to the floor. there doesnt seem to be any leaks and i have look around the net and the closet thing i could find was that the master cylinder is bad. i never have had a car with this problem so i was wondering if anyone else has had this problem. and if a master cylinder could cause this kind of problem.
thanks

i have look around the net and the closet thing i could find was that the master cylinder is badTypical failure for a 20 yr old car!