Apparatus and method for use in testing the fluid tightness of closed vessels or fluid systems, such as found in the pressurized cooling system of an internal combustion engine and the fluid transmission of an automobile. The apparatus is configured to detect and/or quantify fluid leakage from a fluid system that is under pressure, when in use, such as through imperfections in the housing or sealing structure of the fluid system whereby to warn the provide r of such fluid system that a leakage condition exists.
