A B757-200's Center Hydraulic system began losing hydraulic fluid and the EICAS alerted HYD QTY C. A Center system electric hydraulic pump also alerted. An emergency was declared and the flight returned to land at the departure airport.

Narrative

As we were about to level off at FL340 an EICAS alert message HYD QTY - C came on and the center hydraulic quantity on the status page was indicating a quantity of .36 and falling. By the time we had the irregular procedure out the CENTER LEFT HYD ELEC Pump annunciator light came on. As per the C HYD ELEC 1 irregular procedure we turned off the center number 1 pump off; and because we were losing center system hydraulic fluid the HYD QTY - C irregular procedure led us to consider turning off both center electrical hydraulic pumps. Due to the fact we apparently had two separate but related problems; loss of center system hydraulic fluid and center #1 electric pump failure; we chose to leave both of the C electrical hydraulic pumps off for the remainder of the flight. The HYD SYS PRESS - C irregular procedure that a normal approach and landing could be accomplished. Concurrently we had requested clearance from ATC to return to our departure airport. Although a normal landing was anticipated an emergency was declared. Dispatch was notified via ACARS and we returned. Landing performance data was more than adequate for an overweight landing at 214;000 LBS. We had an uneventful smooth minimum decent rate touchdown. After both center electrical pumps were both off the center hydraulic quantity fluctuated from a low quantity of .15 to approximately .48 when parked back at the gate.