What was found
WHILE IN CRUISE FLIGHT WITH THE ENGINE DRIVEN HYDRAULIC PUMPS IN LOW, THE RIGHT HYDRAULIC SYSTEM WENT TO ZERO PRESSURE FOR 3-5 SECONDS THEN RETURNED TO NORMAL 1500 PSI. THIS OCCURRED TWICE AND THE RIGHT HYDRAULIC PRESSURE LOW AND RUDDER CONTROL MANUAL LIGHTS ILLUMINATED. THE AIRCRAFT YAWED AS THE HYDRAULIC SYSTEM WENT BETWEEN RUDDER MANUAL POWERED MODES. FLIGHT CONTINUED AND LANDED SAFELY AT MEM. MAINTENANCE REPLACED RIGHT ENGINE DRIVEN HYDRAULIC PUMP. AIRCRAFT RETURNED TO SERVICE.