What was found
BWI - FLT 2399 - AT FL 210, THE NR 1 ENGINE A-SYSTEM HYDRAULIC LOW PRESSURE LIGHT ILLUMINATED. CAPTAIN SHUT DOWN THE NR 1 ENGINE HYDRAULIC PUMP AND MAINTAINED 3,000 PSI AND ONE-FOURTH QUANTITY IN A-SYSTEM HYDRAULICS. FLIGHT DIVERTED TO BWI AS A PRECAUTIONARY MEASURE AND LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. THE AIRCRAFT TAXIED TO THE GATE ON ITS OWN. MAINTENANCE REMOVED AND REPLACED THE O-RING AT THE A-SYSTEM ELECTRIC HYDRAULIC PUMP ACOUSTIC FILTER. SERVICED RESERVOIR. NO LEAKS NOTED. (X)