Hello,

I had the same EXACT symptoms - hiss sound underneath the dash and fluctuating idle when pressing the brake - many many years ago. In the end I replaced the brake booster using Allied Signal brand (a Bosch subsidiary). All these symptoms went away.

I did the replacing myself. Its a one man job. Taking out the master brake cylinder is possible without dismantling the brake lines. You'd have to pull the master cylinder out (towards front of the car) just enough (bending the brake lines a bit) to facilitate removing the booster. The booster had to be dismantled first on the cabin side of the firewall (plus its connection to the brake pedal).

During installing the new booster don't forget to put back the rubber gasket located between the booster and engine side of the firewall. If not the engine sounds would leak into the cabin. In my case the rubber gasket was compressed thin after all the years. I used RTV to augment the sealing. Installing is reverse of dismantling.

A little adjustment may be needed so that the new booster's rod length to the brake pedal is adequate.

Hope this helps.
Amarin.