VEHICLE WAS PARKED ON A SLIGHT INCLINE IN RESIDENTIAL NEIGHBORHOOD, PARKING BRAKE ENGAGED, 5-SPEED TRANSMISSION IN GEAR. VEHICLE REMAINED STATIONARY FOR AT LEAST 5 MINUTES. SOMETIME LATER (WITHIN PERIOD OF ONE HOUR OR SO) PARKING BRAKE FAILED AND VEHICLE ROLLED DOWN LENGTHY HILL, CROSSING RESIDENTIAL STREET AND CRASHING FRONT INTO A LARGE TREE. CAR WAS DECLARED A TOTAL LOSS.