What was found
DURING CRUISE, HYDRAULIC QUANTITY WENT TO ZERO. FOLLOWED COM PROCEDURE FOR LOSS OF HYDRAULIC SYSTEM. FLIGHT CONTINUED ON TO DTW. YELLOW EMERGENCY WAS DECLARED UPON ARRIVAL IN DTW. AIRCRAFT LANDING WAS UNEVENTFUL. MAINTENANCE REMOVED AND REPLACED RETURN TUBE WITH A TEMPORARY FLEX LINE, SERVICED NR 1 HYDRAULIC SYSTEM, AND SYSTEM LEAK CHECKS GOOD. OPERATIONAL CHECKED GOOD, AND AIRCRAFT RETURNED TO SERVICE.