Abstract: A water line test cap is claimed comprising an elongated body closed at one end and having an elongated cylindrical recess and external threads at the other end thereof, an internally-threaded nut threaded to the elongated body, the nut being provided with a bore communicating with the elongated cylindrical recess and an expandable O-ring disposed therein, the cylindrical recess, the bore and said O-ring being linearly disposed and having the same internal diameter sufficient to permit the entry of a water pipe therein. In a preferred embodiment, the water line test cap is also provided with means to bleed the interior of air.