MECH RPT REGARDING LEAK THAT SHOWED AFTER ACFT AIRBORNE; BUT NOT DURING TEST PROC.

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I WAS INSTRUCTED BY MY LEAD MECH TO WORK ON ACFT. I FIRST READ THE TURNOVER LEFT BY MECH WHO WORKED ON ACFT DURING THE PREVIOUS SHIFT. IT STATED THEY WERE HAVING A PROB TIGHTENING THE B-NUT ON THE HYD FEED LINE TO THE L EDP FILTER MODULE. I RESEARCHED THE SYS IN THE MAINT MANUAL FOR REMOVAL AND REPLACEMENT OF THIS LINE. NO INFO WAS FOUND. I THEN CHKED THE SECTION FOR REMOVAL AND REPLACEMENT OF THE EDP FILTER MODULE AND COMPONENTS. NO MENTION OF THE LINE WAS FOUND IN MAINT MANUAL; SO I USED STANDARD PRACTICES TO REINSTALL THE HYD LINE AND THE B- NUTS WERE TORQUED TO STANDARDS. I THEN LOOKED UP THE PROC FOR LEAK CHKING THE SYS REF. THE HYD SYS WAS PRESSURIZED USING 2 METHODS: 1) USING THE ALTERNATING CURRENT MOTOR PUMP; AND 2) USING THE ENG DRIVER PUMP BY MONITORING THE L ENG WITH THE STARTER UNTIL PRESSURE IS ACHIEVED ON THE L HYD GAUGE. NO LEAK WAS DETECTED USING THESE 2 METHODS. THE ACFT WAS RELEASED FOR SVC ONLY TO RETURN APPROX 45 MINS LATER WITH A LEAK IN THE L HYD SYS. AFTER THE SYS WAS RESVCED AND ANOTHER LEAK CHK WAS PERFORMED THE LEAK WAS STILL NOT EVIDENT. ONLY AFTER THE ENG WAS STARTED DID THE LEAK PRESENT ITSELF. I FEEL THE LEAK WAS REDEVELOPED WHEN THE ENG DEVELOPED TORQUE AND TWISTED OR VIBRATED THE LINE IN THE PYLON. THE LEAK WAS FIXED BY REPLACEMENT OF THE LINE AND UNION AND THE B-NUTS WERE TIGHTENED USING AN ABNORMAL AMOUNT OF TORQUE.