B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew FLC complied w / Automation / Advisory
Aircraft
While at cruise; the EICAS system alerted the crew to a leak in the center hydraulic system.  After landing uneventfully; the airplane remained on the runway for approximately 90 minutes while waiting for a tow.Entire event occurred at night.  Weather upon landing was freezing rain; 2.5 miles visibility; and the runway was contaminated with ice and snow.While in cruise at FL 350 we received a 'C HYD QTY' message on the EICAS system.  We checked the fluid level and it was .48 at that time.  The checklist for this item has no corrective action.  As we watched; the fluid level slowly decreased.  The pressure stayed normal. We reviewed the systems we might lose in case of complete loss of fluid in the center hydraulic system; which include normal gear and flap operation as well as nose wheel steering.   In an attempt to preserve the center system for landing we decided to turn the center system OFF until we were ready to configure for landing.  This appeared to slow; if not stop; the loss of fluid.  Level was .43 when we turned the pumps off.At approximately 4000 feet and on downwind to the runway we decided to turn the center system pumps back on and configure for landing.  When we turned the center pumps back on the level showed .42 but began to decrease again.  Lowering the flaps and gear caused the level to drop dramatically; bouncing off of 0.00 while the gear was in transit; but recovering once the gear was fully down.  Because of the low fluid level we decided to use flaps 25 for landing Instead of flaps 30.  Landing was uneventful.  Because the fluid level showed as low as .14 we decided the stop on the runway and request a tow to parking.Tower was notified while we were on final that we were going to stop on the runway and request a tow.  Tower was again notified after we had safely stopped that we were going to shut down on the runway and wait for a tow.  It took approximately 60 minutes to get a tug to the aircraft; and then another 30 minutes to hook up the tug and get towed clear of the runway.  The tow to the gate; offloading of passengers; and aircraft shutdown was normal and uneventful.The checklist for dealing with The 'C HYD QTY' message only states the obvious; that the fluid is low.   It does not give any guidance towards saving some of the fluid by turning of the hydraulic pumps.  The checklist should have some wording along the lines of 'If the fluid level continues to decrease; turning off the hydraulic pumps should be considered.'
B767-300ER First Officer reported losing the center hydraulic system. An emergency was declared and the aircraft was towed to the gate after a safe landing.