A B767-300 IN CRUISE AT FL310 HAS THE FORWARD FIRST CLASS SINK OVERFLOW INTO THE FIRST CLASS CABIN AND GALLEY CAUSED BY THE DRAIN MAST HEATER CIRCUIT BREAKER PULLED.

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I WAS NOTIFIED BY A FLT ATTENDANT THAT THE L SIDE MID CABIN LAVATORY SINK DRAIN WAS PLUGGED UP. WHEN I INVESTIGATED; I FOUND NO APPARENT REASON FOR THE BLOCKAGE; BUT I SHUT OFF THE WATER SUPPLY VALVE TO PREVENT THE SINK FROM OVERFLOWING. ABOUT 3 HRS LATER; THE MID CABIN LAVATORY SINK ON THE R SIDE STOPPED DRAINING. WHEN WE STARTED THE DSCNT FOR LNDG; THE FORWARD FIRST CLASS LAVATORY SINK OVERFLOWED AND FLOODED THE LAVATORY FLOOR AND THE AISLE FROM THE COCKPIT DOOR TO THE FORWARD ENTRY AND THE FORWARD GALLEY. WHAT A MESS. THE PURSER COVERED THE FLOODED FLOORS WITH SEVERAL BLANKETS PRIOR TO LNDG. AFTER LNDG; MAINT FOUND THE DRAIN MAST FLT CIRCUIT BREAKER OUT. THIS IS THE SECOND TIME THIS HAS HAPPENED TO ME IN THE LAST 12 MONTHS. THE FIRST TIME IT WAS TRACED TO SFO HANGAR MAINT WHO PULLED THE BREAKERS WHILE THE ACFT WAS IN THE HANGAR AND FAILED TO RESET THEM. THIS ACFT SPENT THE PREVIOUS NIGHT IN SFO AND I STRONGLY SUSPECT THAT THE BREAKERS WERE NOT RESET AFTER OVERNIGHT MAINT. ASIDE FROM THE SANITARY ASPECTS OF THIS PROB; IT COULD RESULT IN SERIOUS ELECTRICAL PROBS DURING ETOPS OPS IF THIS MUCH WATER/SEWAGE FLOWS INTO THE ELECTRONICS COMPARTMENTS 180 MINS FROM AN ENRTE ALTERNATE. THE NEXT TIME I HAVE MORE THAN 1 LAVATORY DRAIN MYSTERIOUSLY QUIT WORKING IN THE MID ATLANTIC; I WILL IMMEDIATELY DIVERT TO KEFLAVIC OR LAJES.