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:
http://www.lincolnsonline.com/tech/00021.html. Also check out the Mark VII forum at
www.lincolnsonline.com. Do a search for ABS and you'll find more information than you
ever wanted. There's also a fairly new site: http://www.thelincolnmarkviiclub.org .
Both are fantastic sites with a lot of really good people and a lot of great
information specific to the Mark VII and other Lincolns (I'm biased, I have an 84 and
89 Mark VII).
 It's not a really complicated system but the mechanic needs to know how it works and
what's involved to properly diagnose it. The problem could be as simple as a failed
pressure switch or relay. It could also be as drastic (expensive) as a failed pump,
accumulator, relay and switch. Just a heads up, the accumulators are getting a bit
hard to come by and are fairly expensive, $150-250.

You got charged for a problem fix... it isnt fixed.  Tell "them' that.

Mind, if it's another problem, you may have to pay yet more... but if the
pedal was HARD after the first problem and are soft and cushy, yet have to
push hard to stop, then it seems to me the system needs bleeding to get the
air out.