Edmund,
I have a c3 and have the same problem, I found the exact hose that the hissing sound is comming from, I asked around on the c3 tech forum and I was told it was a valve that takes in vacume for the brake booster, and that it is normal. The problem was that the filter in the "valve" was bad or missing so you hear the hissing!
I am not a mechanic, and sure as heck ain't trying to say anyone is wrong, but just an idea instead of forking out the cash for a new one.
My "valve" looked sort of like the c;licker of a ball point pen, when the pedal is depressed the clicker is out and the hissing sound is heard, then when the brake is let go the brake pedal hiits the "clicker", pushing it in and stopping the hissing sound.
Maybe someone else knows for sure.
GIT R DONE _________________________BORDERBUM