Harry,
there's only so much vacuum volume in your reservoir.  You should only
get one or two soft brake applications with the engine stopped before
the reservoir runs out.  It's just an emergency thing so that if your
engine stops while you're driving, you can safely stop the vehicle.

As for the sound, is it not just the normal power assist hiss?
There is supposed to be some noise from your brake booster when
you apply your brakes.  It's just the sound of the atmosphere-pressure
air in the booster being allowed to leak out to the vacuum.

Cheers

<sstones@io.org>
SStones             Toronto,  Ontario.