AN AIRBUS A300-600 CAPTAIN REPORTS OF NOTICING THE SAME FUEL LEAK PROBLEM FROM THE HORIZ STAB TRIM TANK; ON THE SAME ACFT; THAT HAD BEEN PLACARDED FOR SEVEN DAYS. PILOT REFUSED ACFT.

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A FUEL LEAK ON THIS FLT WAS HANDLED BY PLACARDING THE TRIM TANK ISOLATION VALVE. I FLEW THE FLT TO ZZZ. ON OCT X; SAME ACFT SAME PROB; SAME FLT. MAINT USED SAME PLACARD. I ASKED THE MECH IN ZZZ1 TO GO WITH ME TO CHK THE LINE FOR ANY FUEL LEAKS. THERE WAS FUEL IN THE LINE. MECH STATED THAT THIS WAS RESIDUAL FUEL AND I DID NOT THINK IT WAS AND REFUSED THE ACFT. IT WAS DECIDED THAT FIXING THE PROB IN ZZZ1 WAS THE BEST ACTION. CAN WE SAFELY DEFER A FUEL LEAK? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE FUEL LEAK WAS COMING FROM THE LOWER FUSELAGE DRAIN MAST BETWEEN THE LEFT/RIGHT MAIN LANDING GEAR. APPEARS THIS MAST ALLOWS FOR ANY FUEL LEAKING FROM THE APU OR THE HORIZ STAB FUEL TRIM TANKS ISOLATION VALVE FUEL SUPPLY LINES TO BE CAPTURED INSIDE OF A SHROUD AND DRAIN OUT FROM THIS MAST. THIS ALLOWS FOR LEAK DETECTION. REPORTER ALSO STATED AFTER SEVEN DAYS OF NOTICING THIS SAME LEAK WITH A PLACARD FOR THE APU; AND FLYING THIS SAME ACFT ON DIFFERENT OCCASIONS; AND THE LEAK CONTINUING TO FLOW HE REFUSED THE ACFT. MAINT FOUND A CUT O-RING SEAL FROM THE TRIM TANK ISOLATION VALVE APPEARED TO BE THE SOURCE OF THE LEAK COMING FROM THE DRAIN MAST. REPORTER ADDED HE IS QUITE FRUSTRATED WITH THE DECLINE IN PROPER TROUBLESHOOTING OF ACFT WRITE-UPS. INSTEAD; WHAT HE IS SEEING IS AN INCREASE IN THE RUSH TO DEFER AND PLACARD A SYSTEM BEFORE KNOWING WHAT THE CAUSE OF THE SYSTEM MALFUNCTION REALLY IS. THIS HAS CAUSED SYSTEMS OR COMPONENTS TO BE PLACARDED INOP AND DEFERRED THAT WERE FOUND TO BE ACTUALLY FUNCTIONING PROPERLY.