What was found
FLT 938 - GUM - ON DESCENT INTO GUAM, CREW DECLARED AN EMEGENCY DUE TO 'A' SYSTEM HYDRAULIC QUANTITY LOST. AFTER LANDING, MAINTENACNE CHECKED 'A' SYSTEM QUANTITY WITH 3.5 GALLONS REMAINED. MECHANIC PERFORMED AND EXTERIOR CHECK FOR EVIDENCE OF HYDRAULIC LEAKS AND NONE FOUND. DURING ENGINE RUN UP, MECHANIC COULD NOT PRESSURIZE NR 2 HYDRAULIC SYSTEM. FURTHER TROUBLESHOOT, CONFIRMED NR 2 ENGINE DRIVEN HYDRAULIC PUMP INTERNAL FAILURE. MAINTENANCE REMOVED AND REPLACED NR 2 ENGINE DRIVED HYDRAULIC PUMP ASSY, FLUSHED LINES, REPLACED PRESSURE AND CASE DRAIN FILTERS. AIRCRAFT RETURN BACK TO SERVICE. (W)