89 brakes hissing sound inside car

I have a hissing sound inside the car when I apply the brakes. If I press the pedal real hard it will stop. The brakes seem to work ok but I really dont abuse them that much to tell the difference. What could that hissing sound be and how hard would it be to fix?

its either a brake line vaccuum leak, or the brake booster is going bad, which isnt a fun thing to replace