I noticed the brake was hard to push down so I checked the master cylinder area, no leaks, no noises. Yesterday, I was driving and i noticed an air or hissing noise that started when i pressed the brake and didn't stop until i released it. I am thinking this is a booster problem. Any help would be greatly appreciated.

 

booster is the hiss sound more then likely now u will need to replace the master cyl as well because that is where the brake fluid is leaking into the booster and melting the rubber inside the booster.