If it was my car I would do the following steps

1. Pull off all the wheels and look for leaking calipers or connections
2. Bleed the brakes starting with the right rear,left rear, right front,left front
3. if no solid brakes you could have an internal leak in the master cylinder
4. Replace master cylinder

Repeat step 2.

of course check for pad wear while you are there but that would not produce a soft brake

Post results so we can learn from your experience

__________________
Jerry

2004 SL500 Black 55K Miles
2002 s500 Black 90K Miles
1986 300E Gold 233K Miles
1966 Twin Cessna 320D
1989 Jeep Cherokee 233K Miles
1994 Ford Van E150 399K Miles