What was found
A-SYSTEM HYDRAULICS FAILED IN FLIGHT. IN CRUISE FLT LVL 220, NOTICED A-SYSTEM QUANTITY DECREASING, LEAK CONTINUED TILL CHECKLIST DIRECTED A-PUMPS TO BE TURNED OFF. FOUND O-RING FAILURE AT UPPER PORT OF FLAP CONTROL VALVE. REPLACED O-RING PER 727-200 MM 20-10-011-4. CLEANED AREA, PRESSURIZED AND CHECKED FOR LEAKS, NONE NOTED. TWO GALLONS A-SYS HYDRAULIC FLUID REMAINING, SERVICED HYDRAULICS IAW MM 12-12-000. AIRCRAFT OK FOR CONTINUED SERVICE.