Does it only "Hssssss" when you push in the pedal, when pedal movement stops, then it stops?
Or does it have a continuous "ssssssss" as you're riding the brake?
Older cars, the diaphragms hiss when you press the pedal in. If there is a leak,
vacuum leak, a small one, it will hiss continually. This happens with power brakes
when they get old.
I had a car where the diaphragm went bad around the brake pedal piston, and
made a continuous hisss when my foot was on the brake. When it was out,
there was no hiss, only when I ahd pressure on the brake. It started out as a
momentary Hsssss when I braked, then it for worse over time, till it finally
leaked. Did not effect braking till there was the continuous leak, but I could
still manage... Eventually, I had to disconnect the vacuum hose to the brake
booster, plugged the engine side, and left the booster side open. Thus, I had
manual brakes...
This is an OLDER system, like from a 2000 jetta, yes, but this was what I had when it happened.
If you see the big booster there, you see the pedal piston on the right, center, that
was where the hiss came from... You see the vacuum hose; that was what I
removed, plugged the hose from the motor, and left it open on the booster so
air would still move in and out with the brake,. If you plug that, you would not be
able to brake, or you'd blow out the diaphragm completely, still no brakes...
Here is an MK5 system:
Similar, just a smaller booster, but the setup is the same... Disconnect the vacuum hose
to the booster, and see if you still get the hisss sound, or if it changes. This pic only shows
the hydraulic clutch, but the booster and master are the same. I could not find a pic with
the brake pedal, but the diaphragm is on the interior side of the booster...
Here is a close-up of the booster:
This shows the interior side of the booster, plainly shows the boot, over the diaphragm..
You wanna check there for the hissing...