Hi, a hiss when operating the pedal doesn't always mean a leak (air or vacuum) as what you are doing there is opening a valve and some pressure equalisation is taking place inside the booster chambers, sometimes this is just operationally noisy (especially when the motor is not running). However, sometimes these components do leak, so the best thing to do is carry out some basic operational tests - they only take a minute or two. I found a brilliant PDF about brake booster operation on this site:
http://www.autoshop101.com/forms/brake05.pdf
Download that - its a good read and has simple tests on all aspects of booster operation, etc. Hope it helps.
