It sounds like a leak in your Power Booster. Your brake pedal pushes a plunger in your power booster which strengthens your push then applies it to your master cylinder. The power booster is thebig black round looking thing mounted right on the other side of the firewall from the pedal and isair assist; powered by vaccum. There may be a problem where you over extend the plunger and it causes a vaccum leak ; hissing sound. Check and possibly replace you power booster if you feel this is the problem, there is usually no fixing it.