Bleed the system first starting with right rear, left rear, right front then left front, check for line leaks, if the pedal does not stiffen up you might have an internal leak in the master cylinder, not hard to replace about a 3.5 on an scale of 10

__________________
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