What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT DIVERTED TO BNA. ON GROUND AT MCI. GREEN HYDRAULIC QUANTITY NORMAL (BOTTOM OF GREEN BOX) RIGHT BEFORE LEVEL OFF NOTICED LEVEL REDUCED TO ABOUT A QUARTER OF QUANTITY AND SLOWLY DECREASING EVENTUALLY INTO THE ORANGE BOX INDICATION AND GOT AN ECAM. THE AIRCRAFT LANDED AT BNA SAFELY WITHOUT FURTHER INCIDENT. POST WALK AROUND EVIDENCE OF HYDRAULIC LEAK UNDER #1 ENGINE.MAINTENANCE FOUND THE LEFT ENGINE GREEN HYDRAULIC PUMP DELIVERY PUMP CHECK VALVE LOOSE AND THE O-RING WAS MELTED. REPLACED CHECK VALVE PER AMM 29-11-36 AND ACCOMPLISHED A SATISFACTORY LEAK / OPERATIONAL CHECK.