What was found
FLIGHT 5901, CLE TO MSY - WHILE CRUISING AT FL360 THE FLIGHT CREW RECEIVED A MASTER CAUTION AND EICAS MESSAGE FOR HYD SYS 1 OVHT. THE CREW RAN THROUGH THE APPROPRIATE QRH CHECKLISTS AND SHUT OFF THE SYSTEM. AFTER WAITING THE QRH RECOMMENDED TIME, THE NR 1 HYDRAULIC SYSTEM WAS RE-ENERGIZED. AFTER RE-ENERGIZING THE HYDRAULIC SYSTEM IT WOULD NOT INDICATE AN ACCEPTABLE PRESSURE. ON DESCENT, THE MFD INDICATION FOR THE NR 1 SYSTEM BEGAN TO SHOW PRESSURE, AND DURING APPROACH ALL SYSTEM 1 INDICATIONS WERE NORMAL AND THE HYD SYS 1 OVHT MESSAGE WAS GONE. MAINTENANCE TROUBLESHOT THE HYDRAULIC SYSTEM AND REPLACED THE SYSTEM 1 RESERVOIR AND FILTERS AND THE HYDRAULIC SYSTEM OPERATED NORMALLY. LATER THAT DAY THE HYD SYS 1 OVHT MESSAGE RETURNED. AT THAT POINT THE NR 1 ENGINE DRIVEN HYDRAULIC PUMP WAS REPLACED. THE SYSTEM WAS LEAK CHECKED AND OPERATIONALLY CHECKED AND RELEASED TO SERVICE.