What was found
(CAN) PAN PAN DECLARED DUE TO CENTER HYD HAVING A LOW INDICATION. THE FLIGHT IS NOT EXPECTED TO LAND OVERWEIGHT. ADDITIONAL TIME FOR ALTERNATE FLAP & GEAR EXTENSION IS BEING PLANNED. TOW CREW WILL BE AVAILABLE TO TOW THE ACFT OFF THE RUNWAY AS POSSIBLE STEERING FAILURE MAY ALSO OCCUR. MAINTENANCE:C HYD SYS PRESSURE CAME ON DURING FLIGHT RESOLUTION: FOUND O-RING PERISHED ON MLG SELECTOR VALVE, MAJOR LEAK. O-RING REPLACED AND LEAK CHECKED IAW AMM 32-31-02, PB401.