What was found
PILOT REPORTED AFT SINKS (GALLEY AND LAV) WOULD NOT DRAIN. T/S SYSTEM, FOUND DRAIN MAST NOT HEATING UP. REMOVED MAST AND INSTALLED NEW MAST OF SAME P/N, S/N 396. PERFORMED OPERATIONAL CHECK OF DRAIN MAST HEATING ELEMENT, NO DEFECTS NOTED. SINKS WERE NOT DRAINING DUE TO MAST FREEZING UP AT ALTITUDE, SINKS NOW DRAIN PROPERLY IN-FLIGHT. ALL WORK PERFORMED IAW GIV MM. ACFT TT: 3,388.2 HOURS.