How can I know if my brake booster is bad?

What is your question?

My grandson replaced Calipers and pads on front. Had both off at the same time so a good amount of fluid loss occurred. Im sure a lot of air got into system. We bled the system but still no pressure after just a few seconds after pumping the pedal. I read where the ABS module may have to be burped. I tried the old fashion way because I have a long gravel way and can safely get to speed and cause abs to engage without killing myself. (its long). Did this several times and then get a little more air out of lines doing all four wheels. Repeated this whole process several times. When I pump up the brakes they will get solid and hold pedal but if I start the engine it goes to floor. Even with engine off after I have built pressure and I let off for 30 seconds the pedal will go to floor again. I am hearing a hissing sound upon pressing pedal as well as letting off pedal in the brake booster. Is that my issue now? Quite coincidental if so.... Thanks

How long has your 2004 Dodge Ram 1500 had this problem?

three days