What was found
INFLIGHT THE WING TANK OVERFILL L CAS MESSAGE CAME ON. THE LEFT WING TANK SHOWED 3200 LBS OF FUEL IN IT AND FUEL STARTED TO VENT OVERBOARD, VENTING STOPPED AT ABOUT 2970 LBS INDICATED IN L WING TANK. AFTER LANDING (20 MIN LATER) CREW NOTICED THE UNDERSIDE OF THE L WING WAS WET. DEFUELED & SUMPED LEFT WING TANK. GAINED ACCESS TO AND INSPECTED CLIMB VENT TUBE AT I/B END OF WING TANK & FOUND IT TO BE POSITIONED & SECURED PROPERLY. INSPECTED SURGE TANK DRAIN LINE FOR CLOGS WITH BOROSCOPE & FOUND NO DEBRIS. REPLACED SURGE TANK DRAIN FLAPPER VALVE P/N 9914475-1 WITH NEW FLAPPER. REPLACED VENT FLOAT VALVE AT O/B END OF WING TANK WITH A SERVICEABLE UNIT. REFUELED WING TANKS & FOUND NO LEAKS.