EE WAS OPERATING A FABCO RENTAL D250E HAUL UNIT THAT DEVELOPED ENGINE FAILURE WHILE TRAVERSING A HILL. IT BEGAN TO ROLL BACKWARDS, PARKING BRAKE WAS ACTIVATED BUT DID NOT HOLD. IT WAS STEERED INTO BANK STOPPING MACHINE ABRUBTLY WHEN IT HIT SAND PILE. HE UNHOOKED SEAT BELT AND CRAWLED OUT OF D250E. PRESHIFT EXAMINATION WAS COMPLETE, ALL SYSTEM CHECKED OUT O.K.