Abstract: A flow control valve has a ball and an annular seat providing a positive stop for automatically preventing flow through the valve when a line downstream has been broken. The flow control valve is useful for interior applications in which seepage through the valve would be unacceptable and in which both hot and cold water lines need to be protected. A magnet may aid in positioning the ball. Sensors may be used to detect positions of the ball. A flow control system that includes the flow control valve may include any of a variety of interchangeable components for facilitating incorporation of the flow control valve in a water or other fluid line. Another embodiment includes a mechanism for detecting and stopping flow due to slow leaks.