What was found
(CAN) DURING FLIGHT, THE CREW NOTICED THE HYDRAULIC SYS PRESSURE INDICATION WAS ZERO AND A LARGE QTY OF HYDRAULIC FLUID WAS VISIBLE IN THE CENTER CABIN AREA DRIPPING FROM THE CABIN CEILING. ACFT RETURNED TO BASE FOR MAINTENANCE. INVESTIGATION REVEALED THAT THE HYDRAULIC FLUID WAS LEAKING OUT OF THE FLAP SELECTOR LOCATED IN THE CABIN ROOF CENTER SECTION. FLAP SELECTOR WAS REPLACED AND HYDRAULIC SYS NOW OPERATING NORMALLY.