It would help if you let us know what year and model you are talking about.

I'm betting it is a vacuum leak in the vacuum booster, possibly from a bad diaphram. Does it hiss whenever you have your foot on the brake, but not hiss anytime else? Does the pedal feel different since the hissing started?

__________________
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.