What was found
INFLIGHT PROGRESSIVE FAILURE OF ONBOARD POTABLE WATER SYSTEM TO INCLUDE LOSS OF PRESSURE AND WATER FLOW IN ALL GALLEYS AND LAVATORIES EXCEPT AFT GALLEY AND 4 L/R LAVS. RESERVICED WATER SYSTEM. MANUALLY PLACED PRESELECT OVERFLOW VALVE IN THE OPEN POSITION FOR SERVICING. SUSPECT VALVE INTERMITTENT IN INOP POSITION FWD VALVE TO MCO AND PLACARD PER MEL 38-10-01A. DISCONNECTED AND STOWED VALVE ELECTRICAL CONNECTOR. ALSO DURING T/S FOUND L-4 ISOLATION VALVE NOT IN THE FULL OPEN POSITION. REPOSITIONED VALVE WATER SUPPLY, NOW NORMAL.