How to check if a brake booster is good or not?

I have the opposite problem with my Shelby Dakota. Brake pedal is good and solid/hard when it is not running and goes to the floor when i start it up..... bled and re-bled them. Abslolutely no air in the lines...... I am thinking bad booster. I can hear it hissing at me from under the dash when i press the pedal with it running.