When I remove my foot from my brake it makes a hissing sound..why?

The vacuum assisted brake booster has an internal seal that's leaking 'atmosphere' and the booster should be replaced. Brake operation may not be affected but engine idle 'might' be! Now, the danger here is possibly no assisted service brake aplication if the engine stalls!! Check the estimator here for a price in your zip code. To learn more on how the brake booster works, goggle it! I find that many mechanics really dont know exactly how they work either! I didn't know until i took a course in brake systems, some years ago. Not a lot has changed really.