What was found
THE HYD SYS 3 ELEVATOR OFF LIGHT ILLUMINATED. THE NR 3 PRESSURE AND QUANTITY WERE NORMAL. APPROXIMATELY 2.5 HOURS LATER THE SYSTEM 3 PRESSURE LIGHTS CAME ON. BOTH PUMPS WERE TRIED, BUT THE PRESSURE STILL DROPPED WITH THE QUANTITY INDICATING 7 GALLONS. ALTERNATE GEAR EXTENSION WAS USED AND THE AIRCRAFT LANDED WITHOUT INCIDENT. THE NR 3 HYD SYSTEM WAS FOUND EMPTY. MAINTENANCE FOUND UNION LEAKING AT 2 TO 3 REV PUMP. THE O-RING AT THE UNION WAS REMOVED AND REPLACED. IN ADDITION, THE NR 3 HYD QUANTITY GUAGE WAS REMOVED AND REPLACED DUE TO FAULTY OPERATION. THE NR 3 HYD SYSTEM WAS SERVICED. LEAK AND OPERATIONAL CHECKS WERE GOOD.