Sounds like bad booster! The push-rod for the booster is oftentimes prone to damage during installation, breaking the diaphragm. Also might try pulling vacuum hose off motor and then pulling check-valve out of booster to see if it's holding stored vacuum in the booster. Also need to check adjustable push rod from booster diaphragm to master for correct adjustment.
Most folks don't want to be guessing about somebody else's brake problem. You're probably only going to be given advice for not-in-motion testing.....Good luck!