What was found
(CAN) AFTER TAKEOFF WITH THE LANDING GEAR AND FLAPS UP, THE HYDRAULIC QUANTITY WAS 3.0 GALLONS. ONE HOUR INTO FLIGHT HYDRAULIC QUANTITY WAS 1.8 GALLONS. SEVENTEEN MINUTES LATER QUANTITY WAS 1.5. QUANTITY DROPPED 1.4 GALLONS IN 10 MINUTES. AIRCRAFT DIVERTED. ON APPROACH GEAR AND FLAPS UP QUANTITY 1.0 GALLONS, FLAPS/GEAR DOWN QUANTITY AT 0.8 GALLONS. HIGH PRESSURE FILTER 'O' RING REPLACED 'A' SYSTEM.