Abstract: A flow control device for use in a fluid or gas line is disclosed. In one example, the device controls the flow of air to the brakes of a large vehicle such as a semi-trailer truck. The device includes a housing having an inlet port and an outlet port both of which communicate with the interior of the housing and a moveable valve assembly which selectively can close or open the ports with respect to one another. The valve assembly includes means for axially moving the assembly and locking it in open and closed positions. Means are provided for biasing the valve assembly to an open position to prevent accidental closing of the valve which possibly could result in the application of brakes of the vehicle.