The most common cause of this problem is a failed accumulator. It's the black "ball" attached to the ABS unit. If the entire assembly was replaced with a used one with a failed accumulator then you will have the same problem. There is also the pump motor and it's circuit to consider, including a relay and a pressure switch. Often failures on this system are progressive. The accumulator starts to fail causing the pump to run too long and too often shorting the pressure switch. This eventually causes the pump relay to stick closed allowing the pump to run until it self destructs. If the relay burns up and no longer closes the pump won't be damaged. Problems in any of these areas can cause a hard pedal. There is a Tech article that gives some insight into the Mark VII ABS system here: