Abstract: A check valve having a body with an inlet and an outlet which are coaxially aligned and a valve element including a section of a sphere which is spring loaded urging the valve element towards and sealing against the inlet. The valve element is supported on a stem which is coaxially movable in the valve body. The body has a cavity of a cross-sectional area greater than the valve element by an amount at least as great as the area of the inlet whereby the valve avoids restricting the flow of fluid through the body. Locking means are provided for locking the valve element in the closed position.