I replaced front brake pads, and now hear a hissing sound behind the pedal.

What is your question?

The pedal goes all the way to the floor and my wife is afraid to drive it. I have been changing my own brake pads and shoes for many years on many vehicles and have never had this before. I am not sure if it is the booster or the master cylinder, or if I screwed up the reinstallation of the calipers. Does anyone know how to check these things. The booster looks like a real pain in the A to get to and I think it may have sprung a vacuum leak or something when I squeezed the caliper piston in to accommodate the new pads. Help Please???