EMERGENCY PARKING BRAKE FAILED WHILE PARKED ON A HILL IN A PARKING LOT. WITNESS HEARD A LOUD "POP" AND WITNESSED THE VEHICLE ROLLING DOWN THE PARKING LOT AND STRIKING ANOTHER VEHICLE. WHEN I ARRIVED AND SEPARATED THE VEHICLES, I PLACED THE PARKING BRAKE IN SERVICES AND EXITED THE VEHICLE. AS SOON AS I TOOK TWO STEPS FROM THE VEHICLE I HEARD A LOUD POP, AND THE PARKING BRAKE HAD RELEASED AGAIN. *AK