Brake booster question

If a Mopar booster (photo would help), there are vacuum seals on both the input (sliding O-ring) and output (rubber bellows) sides. That is even true in 1990's cars (Breeze, Intrepid). There are thin pancake style boosters which seal vacuum on the output side via a rubber seal at the MC. My 1980's M-B and (I think) Mopar minivans are like that. Allows a thinner booster, but any brake fluid leaks get sucked into the booster. I agree that your booster sounds defective. You usually hear the air hissing in, most often from the cabin side where the brake rod goes in. Your engine would also idle faster from the increased air flow, except in modern engines where the IAC valve compensates to maintain rpm.