WATER OVERFLOWED IN FWD LAV OF A B767-300. MAINT WAS CONTACTED AND INSTRUCTIONS WERE RECEIVED FOR LOCATING SHUTOFF VALVE. NO ELECTRICAL EQUIPMENT WAS AFFECTED.

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THE FORWARD AFT LAVATORY JUST AFT OF DOOR 1L OVERFLOWED. THE RINSE WATER WOULD NOT SHUT OFF AND SPILLED APPROX 15-20 (ESTIMATED) GALLONS OF WATER ALL OVER LAVATORY FLOOR AND FIRST CLASS CARPET. WATER WAS 1-2 INCHES DEEP IN LAVATORY AND CARPET WAS SOAKED IN ALMOST ALL OF THE SURROUNDING SEATS. WE SATCOM MAINT AND HE ADVISES TO REMOVE TOILET SEAT AND SHROUD; THEN FIND SHUTOFF VALVE AND CLOSE. THIS PROCESS TOOK APPROX 15 MINS TO FINALLY LOCATE THE SHUTOFF VALVE ALL THE WHILE WATER STILL FLOODING. FLT MANUAL GIVES NO MENTION OF A SHUTOFF VALVE OR ITS LOCATION BEHIND TOILET. A FLT MANUAL DIAGRAM WOULD HAVE BEEN HELPFUL. SATCOM SAVED THE DAY AS WE COULD COM WITH MAINT AND HE TALKED US THROUGH. IF IN A B757 OVER HF RADIOS; THIS WOULD HAVE BEEN VERY DIFFICULT. ONCE WATER WAS SHUT OFF WE HAD PASSED ETP AND DISCUSSED WITH MAINT THAT THERE HAVE NOT BEEN ELECTRICAL ANOMALIES DUE TO WATER SPILLS. WE CONTINUED WITHOUT FURTHER PROBS. THERE HAD BEEN 2 PREVIOUS WRITE-UPS OF 1 WK AND 2 WKS EARLIER OF THE SAME PROB IN THE SAME LAVATORY.