Hiss when I press the brakes

When I am at idle and press the brake pedal, I hear a hiss. The brakes work fine. I am guessing this is a vacum leak. Has anyone had this problem, and where was the leak? This is on a '87 XJ.

BrianC

My 87 does the same thing. It is the brake booster. My hisses but I have done a vacuum test and it seemed to do ok. My brakes also work fine. They feel like my 2000XJ with none ABS. This is of course when I get under the back and adjust the "self adjusting brakes"

8700XJS