What was found
PHL-FLT 1451 - EN ROUTE FROM MHT-DCA, AT FL350, A SYS HYD FLUID DROPPED TO 1/4 AND THE LOW PRESS LIGHT FOR THE NR 1 ENG HYD PUMP ILLUMINATED. CREW ACCOMP QUICK REF HANDBOOK PROCEDURES AND SHUT OFF THE PUMP WITHIN 20 SECONDS. FLUID LVL CONTINUED TO DROP TO 1/8 AFTER 4/5 MINS. A SYS HYD PRESS WENT TO 0 PSI BUT FLUCTUATED TWICE. THE ELECT PUMP LOW PRESS LIGHT ILLUM IMMEDIATELY THEN FLICKERED. CREW FINISHED CHECKLIST AND TURNED OFF A SYS ELECT PUMP IN LESS THAN 10 SECS. FLT DIVERTED TO PHL, LANDING W/O INCIDENT. TOWED TO GATE AND DECLARED EMERGENCY. MX FOUND THE O/B CHECK VLV ON THE A SYS HYD PRESS MODULE UNSCREWED AND O-RING BLOWN. INSTALLED NEW O-RING AND BACKING RING AND TIGHTENED CHECK VALVE.