If the problem isn't in the master cylinder it coiuld very well be the rubber hose for the rear brake line, collapsed internally. The parking brake is a completely separate system on these cars and not likely involved in the matter (the park brakes are purely mechanical not hydraulic).
'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.