Did you install a new or reman'd master cylinder?
Still sounds like a bad master cylinder to me. Also, did you bench bleed the master before installing? I have read that the MB masters need to be bench bled prior to installation to ensure proper performance.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.