What was found
ON DEPARTURE WHILE CLIMBING THROUGH 9000-10000 FEET THE CREW NOTICED A LOW HYDRAULIC FLUID MESSAGE. WHEN THEY CHECKED THE HYDRAULIC FLUID LEVEL THE CENTER SYSTEM WAS DROPPING RAPIDLY AND WITHIN ABOUT 5-10 SECONDS WAS AT ZERO. ATC DECLARED AN EMERGENCY ON THE CREWS BEHALF. CREW AIR TURNED BACK TO CVG (ATB) AND LANDED WITHOUT INCIDENT. REMOVED AND REPLACED CENTER SYSTEM AIR DRIVEN PUMP PRESSURE/CASE DRAIN FILTER MODULE IAW 29-11-19. OPS CK LEAK CK GOOD. A/C RETURNED TO SERVICE.