What was found
CREW REPORTED, NR 1 FUEL TANK WENT 500 POUNDS LOWER THAN NR 3 TANK. BALANCED FUEL, BUT IMBALANCE OCCURRED AGAIN. *S/D* THE NR 1 FUEL TANK VENT ELBOW CRACKED. REPLACED THE VENT ELBOW. SEVERAL OCCURENCES OF THIS DISCREPANCY HAVE BEEN REPORTED. FAILURE HAS BEEN ATTRIBUTED TO RESTRICTED VENT LINE MOVEMENT FROM LINE SUPPORT CLAMPS INSTALLED DURING MAINTENANCE. A SPECIAL PROJECT TO CHECK ALL AFFECTED AIRCRAFT IS BEING ACCOMPLISHED.