What was found
IAD/EWR - FLT 7544 - DURING TAKEOFF, AIRCRAFT MADE AN UNSCHEDULED LANDING DUE TO FLAPS WOULD NOT RETRACT FROM 9 DEGREES. AIRCRAFT RETURNED TO GATE WITH NO FUTHER PROBLEMS. MAINTENANCE INSPECTED AND FOUND BYPASS PIN POPPED ON HYDRAULIC RETURN FILTER. MAINTENANCE REMOVED AND REPLACED RETURN FILTER AND FLUSHED SYSTEM AND RESET PIN AND CYCLED FLAPS THROUGH 40 TIMES. OPS CHECK OK. AIRCRAFT WAS RELEASED AND APPROVED FOR RETURN TO SERVICE. (X)