I have to agree with everyone above. The hiss when you press it brake pedal is a torn diaphragm in the vacuum booster. When you do not have the brakes pressed, the diaphragm is "at rest" and the tear is sealing itself. When you depress the pedal, it stretches the diaphragm and the tear opens When this happens, all the stored vacuum pressure is released through the tear instead of pulling on the diaphragm to assist in braking power. I had an 83 that made that exact sound, woosh, hard pedal - stand on the pedal for all you are worth to get it to stop. REPLACE THE BOOSTER!!!! IT HAS FAILED. The sinking pedal after the woosh could be the wheel cylinders, or a bypassing master cylinder. But since you do have a wheel cylinder leak, replace them and the sinking pedal should stop. When I had a master bypass (it was in an E-150 van, but same basic braking system) the pedal hit the floor and the brakes did nothing..... so I would say it is the leaking cylinder is causing the sinking pedal.
