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?
'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.