|While pressing on the brake with the booster hooked up and you're hearing a hissing sound? most likely a bad booster:
Hook it back up, with your foot on the brake you should feel the pedal drop slightly when the engine starts. If it doesn't that's more proof the booster is bad or the check valve.
Let the engine run a minute or two, turn it off. Slowly Press the brake pedal several times. You should feel pedal sink low the first time and then get hard to press and higher. If it doesn't, another sign of a bad booster/check valve.