Hissing Sound until brakes are applied - 1987 Audi 5000 Quattro

[Dave Marshall]

Hi All

New problem today.

With the engine running, there is a hissing type of sound coming from near
the top of the brake pedal.  Step on the brake pedal and the sound goes
away.

I have the Bentley for my car, but I can't seem to find where there may be a
vacuum hose or what ever that is leaking.

I have dug around in there, but can't find where the leak is as of yet.  It
is definitely right close to the brake pedal.

Any ideas?

Thanks