'92 Park Avenue brakes problem

I have a '92 Park Avenue with the 3.8 engine and 119K miles. Drives like a dream, but it has recently developed a fading brake problem. Only when stopped, transmission in gear, brake applied, does the pedal *slowly* go to the floor. It can be pumped up, but does it again, each and every time.
A new master cylinder has been installed, actually twice, to no avail. No leaking fluid has been found on the pavement.
Need ideas as to the problem/fix. What vacuum or booster checks can I do? Flush and fill? I need brakes!
I can't have the wife drive the car as it's unsafe. It will creep away at a stop light if you don't reapply the pedal. Thanks ahead of time!