What was found
MDT-FLIGHT 3963. CREW REPORTED LOSS OF HYDRAULIC SYSTEM AND NR 1 QUANTITY IN FLIGHT WITH RUDDER ISOLATION VALVE LIGHT, FOLLOWED SHORTLY AFTER NR 1 HYDRAULIC PUMP, NR 1 HYDRAULIC PRESSURE, RUDDER PRESSURE AND FULL RUDDER PRESSURE LIGHTS ILLUMINATED. ZERO QUANTITY IN SYSTEM, RAN FOR APPROXIMATELY 14 MINUTES. MAINTENANCE INSPECTED AND FOUND CUT O-RING ON NR 1 CASE DRAIN FILTER ASSEMBLY. PACKING REPLACED. OPERATIONAL AND LEAK CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.