Check the main vacuum line running from the vacuum pump to the brake booster. If no obvious cracks/breaks/leaks, suggest testing the vacuum pump. You should see something like 20-22 inches HG from the pump (dieselheads, correct me if I'm wrong).

__________________
Mike Tangas
'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.