What was found
FLT 4484 - WHILE DESCENDING THROUGH FL310, FROM DAL TO ATL, A 'HYD 1 LOW PRESS' MESSAGE WAS RECEIVED BY THE PILOT. THE HYDRAULIC QUANTITY SHOWED ZERO. THE PILOT SLOWED DOWN TO 250 KNOTS PER QRH PROCEDURES AND DECLARED AN EMERGENCY. AIRCRAFT LANDED SAFELY IN ATL. MAINTENANCE NOTED A TOTAL LOSS OF HYDRAULIC QUANTITY. MAINTENANCE REMOVED AND REPLACED THE NR 1 ENGINE DRIVEN HYDRAULIC PUMP DUE TO IT BEING RUN DRY FOR APPROX 37 MINUTES. MAINTENANCE ALSO REPLACED THE NR 1 HYDRAULIC PRESSURE TUBE WHICH WAS DETERMINED TO BE THE SITE OF THE LEAK. LEAK AND OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE. (M)