What was found
WHILE IN DESCENT PROFILE, BELOW 18,000 FEET, THE FLIGHT CREW NOTICED THE "L HYD QTY" MESSAGE FOLLOWED BY THE RESERVOIR LIGHT FOR THE LEFT HAND HYDRAULIC SYSTEM. THE LEFT RESERVOIR QUANTITY WAS BELOW 50%. THE QRH WAS COMPLIED WITH AND AN EMERGENCY WAS DECLARED WITH ATC AT ATL. THE AIRCRAFT LANDED UNEVENTFULLY. CORRECTIVE ACTIONS: MAINTENANCE FOUND THE ALTERNATE BRAKE SELECT VALVE IN THE RIGHT WHEEL WELL WAS LEAKING AND REPLACED IT, PER B-757 AMM 32-41. NO OTHER LEAKS WERE NOTED, ALL CHECKS WERE GOOD, AND THE AIRCRAFT RETURNED TO SERVICE.