What was found
DIVERSION, IN CRUISE LAVATORIES 4L/R, 3 L/R, AND AFT GALLEY SINKS BACKED UP WITH WATER. UNABLE TO MANUALLY SHUTOFF WATER TO AFT LAV'S. FLOODING PERSISTED IN AFT GALLEY THROUGH DESCENT, APPROACH, AND LANDING. BLOCKAGE CLEARED, REMOVED AND REPLACED AFT DRAIN MAST IAW AMM 30-71 OPS CHECKED GOOD.