UNABLE TO PRESSURIZE POTABLE WATER TANK.

Narrative

WHILE BEING PUSHED BACK AT DFW; THE GND CREW INFORMED ME THAT WATER WAS LEAKING FROM THE POTABLE WATER SVC PANEL. I ASKED THE GND CREW TO CHK THE PANEL VALVE. SHORTLY THEREAFTER I WAS INFORMED THE PANEL WAS NORMAL. EVERYTHING PROCEEDED NORMALLY UNTIL WE WERE INFORMED BY THE #1 FLT ATTENDANT THAT SHE COULD NOT MAKE ANY COFFEE; THIS WAS AT APPROX 24000 FT. ON THE CLB TO 33000 FT. THE LAVATORY SINKS ALSO WOULD NOT PRESSURIZE. WE SUSPECTED THE WATER SVC PANEL VALVE WAS IN THE WRONG POS AND THAT WE MAY BE VENTING WATER OVERBOARD; I DECIDED TO DIVERT THE FLT TO A STATION WHERE WE COULD CHK THE VALVE; AND DISPATCH DECIDED TO RETURN US TO DFW. MY MAIN CONCERN WAS THAT WATER COULD FREEZE AFT OF THE SVC PANEL AND THEN BREAK LOOSE AND FOREIGN OBJECT DAMAGE THE L ENG. WE DID NOT HAVE ENOUGH FUEL TO FLY TO YYZ BELOW THE FREEZING LEVEL; SO I FELT A DIVERT WAS NECESSARY. WE DSNDED BELOW THE FREEZING LEVEL AND RETURNED TO DFW WHERE THE VALVE WAS FOUND IN THE VENT POS. NO FOREIGN OBJECT DAMAGE WAS FOUND. THE ACFT WAS SVCED AND WE PROCEEDED UNEVENTFULLY TO YYZ. THE NEXT TIME THERE IS ANY DOUBT ABOUT THE VALVE POS; I WILL HAVE THE LAVATORY. SINKS CHKED FOR WATER FLOW BEFORE LEAVING THE RAMP.