On June 17, 2000, about 0612 Pacific daylight time, a Douglas DC-8-73, N831AL, registered to Wilmington Trust Company, and operated by Emery Worldwide Airlines (EWA) as a 14 CFR 121 supplemental cargo flight, declared an emergency subsequent to a failure of the aircraft's systems. Visual meteorological conditions prevailed and an instrument flight plan was filed. The aircraft landed at it's planned destination, Seattle-Tacoma International Airport, Seattle, Washington, without further incident. There were no injuries, no aircraft damage and no report of fire. The flight originated from Dayton, Ohio, approximately 3 1/2 hours prior to the incident. The flight crew reported that while in cruise flight at flight level 310 (31,000 feet) they noted a loss of hydraulic fluid quantity in the aircraft's main hydraulic system reservoir. Shortly after powering the auxiliary rudder pump, the crew felt a "...chattering and feedback with the rudder pedals". The crew also reported that the rudder momentarily deflected to the left. After de-powering the system, the flight crew declared an emergency with Seattle Air Route Traffic Control System (ARTCC) and landed without further incident. Maintenance personnel from EWA reported that a post flight inspection of the aircraft's hydraulic system indicated that the auxiliary hydraulic pump was leaking fluid. They also reported that the hydraulic fluid quantity was at "standpipe level". On June 30, 2000, representatives from Aero-Craft Hydraulics, Inc, and the Federal Aviation Administration, inspected the auxiliary hydraulic pump assembly. Examination of the assembly revealed that a gasket (part number MS9134-01), located between the hydraulic pump mainframe and the AC electric drive motor, had failed. Further examination of the hydraulic pump subassembly revealed that the rate piston (part number 56909) was "frozen" in the extended position. Heavy scoring on the piston and the piston housing, with evidence of contamination were also noted. Representatives from Aero-Craft Hydraulics reported that a frozen rate piston (in the extended position) could result in high internal case pressures capable of exceeding the manufactures specifications and the specifications of the gasket (Report Attached).