Brake Power booster

You should try to locate the source of the hissing before you replace anything. There's a vacuum line and a valve attached to the brake pedal (at least there was on my 1987 GT) for some purpose or another, perhaps that's the source of the hissing. Failing that, it's probably a vacuum leak from the brake booster where it meets with the inside of the car... that certainly would be logical, given the symptoms you describe. That you are able to stop the car at all makes me think the master cylinder is fine, plus there's that hissing sound which is presumably a vacuum leak. That could cause the brake booster to fail, leaving you with excess pedal effort in order to make the vehicle stop. As such, I'd put money on it being a failed brake booster and the master cylinder being just fine.

Its a secondary disabling switch for the cruise control.