What was found
APPROX. 1 HOUR INTO FLIGHT LEVEL 1 HYD EIU 3 OFF ALERT. HYD QTY 3.2. HYD QTY CONTINUED TO DECREASE TO 2.5 LEVEL 1 HYD 3 OFF. QTY THEN HELD SEADY AT 2.5. AT SLAT EXNTENSION, QTY IMMEDIATLY DROPPED AND RECIEVED LEVEL 2 HYD FAIL ALERT. QTY WENT TO NEAR 0. REMOVED AND REPLACED THE CHECK VALVE THAT WAS LEAKING ON THE UPPER HYD TRIM VALVE PER M/M 27-43-41-4-0. PERFORMED LEAK CHECK ON VALVE AND ALL SYSTEM ARE GOOD. THE ORING WAS FOUND TO BE SPLIT ON OLD CHECK VALVE.