Most of the time this is a indication of a bad power brake booster. The booster is operated by applying vacuum from the engine, when the booster begins to leak you hear that vacuum leaking inside the car. Your certified service center can check this and let you know what it will take to fix. This booster is a major part of the braking system so if you hear this hissing you should have it checked right away.
#booster #brake #hissing