Loss of Brake pedal when backing up

[19k40rocky]

Got a new problem with my daughter's car now. When you start it up and then back down the drive way (its kind of on a hill) to the street, the brake pedal goes out from underneath your foot and you can hear the ABS chattering on the other side of the firewall. It doesn't do it anyother time, just when backing it up after start up. It doesn't have any trouble codes either. Anyone got any ideas? All help is very much appreciated.