Think about this question a minute........ what other action are you doing besides backing up? Turning the steering wheel? There is no way for the brakes to act any different in reverse than in drive, UNLESS when you turn the front wheels, A caliper can be hitting something which moves it out of position and lets the piston travel further, then the pedal drops!! Had this happen on a Chevy and the steering stop was broken letting the wheels turn too short to the right when turning around and backing up. Sounds crazy but it happened to me. Took me forever to figure this one cause it was ok when going STRAIGHT foward or backward. This is just one problem that is possible, i don't think anything else is. IT IS NOT THE BOOSTER!! YOU are doing something different in reverse than in drive, are you turning around when this happens? Think about it!! Let me know what you find i can tell you how to test this further, got to go now!