Flight Crew Air Transport Pilot (ATP)
Other / Unknown
Aircraft Equipment Problem Less Severe
Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
Coasting in over ZZZZ I had just returned to the cockpit after using the restroom. The Captain (who just returned from his break) and first officer were discussing the EICAS Hyd Low Quantity for the Center Hydraulic system.  The Status page indicated that the Center Hyd quantity was now at .47 or 47 percent.  We referred to the QRH HYDRAULIC QUANTITY checklist which simply stated that the quantity was low and that was the end of the procedure.Watching the quantity over a short period of time; it was apparent that the system was continuing to lose fluid.  It was also apparent that we could possibly need to accomplish the loss of HYDRAULIC SYSTEM PRESSURE.  Loss of this system would have meant the loss of center auto pilot; some spoiler panels; center stabilizer trim; normal flap operations; normal gear extension/retraction and no automatic speed breaks.  Maintenance was contacted via SATCOM; it was suggested that removal of system pressure by turning off all pumps we might retain C Hyd fluid for use during approach and landing.  All three pumps were selected off and the quantity stabilized at 43 percent.  We also confirmed that if we lose center system pressure that once we extend the gear; we will not be able to retract it.We had reviewed all applicable landing; climb and cruise charts and QRH procedures for a loss of center hydraulics pressure if it became necessary.   We coordinated with dispatch our intentions to continue. We requested a 25 mile final to configure the aircraft for landing. Once established on the long final we reselected the pumps back on. Configured to land or C System quantity was down to .13 or 13 percent. System pressure was maintained throughout the approach and landing and we were able to make a precautionary landing.We taxied to the gate with [Airport Rescue and Fire Fighting (ARFF)] following with no other complications. After landing/parking checks complete the quantity returned to a .31 or 31 percent.
B767 First Officer reported they slowly lost the center hydraulic system quantity; but landed uneventfully at their destination.