EMPLOYEE WAS OPERATING KOMATSU HAULAGE TRUCK, DRIVING UP AN 8% GRADE WHEN THE ENGINE STOPPED. THE PARK BRAKE WAS SET AND THE EMPLOYEE WAS ABOUT TO RESET THE BATTERIES WHEN THE TRUCK ROLLED BACK, CAUSING THE EMPLOYEE TO LOSE HIS BALANCE AND FALL NINE FEET DOWN TO THE BUMPER DECK, FROMWHICH HE JUMPED TO THE GROUND.