What was found
HYD LINE WAS REPLACED DUE TO CRACK IN FLARE AT TEE FITTING. REPLACEMENT OF LINE REQUIRED INSTALL OF NEW O-RING, WHERE TEE FITTING ATTACHES TO FLIGHT CTRL HYD SHUT OFF VALVE. AFTER INSTALL OF HYD LINE AND O-RING, A LEAK CHECK WAS PERFORMED. NO LEAKS WERE NOTED. AC WAS APPROVED FOR RETURN TO SERVICE. AFTER 2.6 HOURS, O-RING FAILED CAUSING LEAKAGE OF HYD FLUID FROM COMBINED HYD SYS. AC MADE PRECAUTIONARY LANDING. MAINT REPLACED DEFECTIVE O-RING AND SERVICED HYDR SYS. LEAK CHECK WAS PERFORMED AND NO DEFECTS WERE NOTED. DEFECTIVE O-RING WAS INSPECTED AND FOUND TO BE CUT. DETERMINATION ALSO COULD NOT BE MADE AS TO WHY O-RING FAILED 2.6 HOURS AFTER INSTALLATION, INITIAL LEAK CHECK SHOWED NO DEFECTS.