ASSOC. GOT IN PICK UP PUT IN REVERSE, RELEASED PARKING BRAKE, PARKING BRAKE DID NOT DISENGAGE ASSOC. GOT OUT OF TRUCK (STILL IN REVERSE) HOOKED CABLE BACK ON PARKING BRAKE. BRAKE DISENGAGED TRUCK STARTED MOVING BACKWARD. DOOR OF TRUCK STRCK ASSOC. KNOCKING HIM DOWN & FRONT WHEELS OF VEHICLE ROLLED OVER FOOT AND ANKLE.