Abstract: A motor vehicle security system for trucks, trailers and the like equipped with conventional fluid operated braking assemblies. The security system comprises a manually operable valve for controlling the application of fluid under pressure to the braking assemblies to manually set the vehicle brakes. The valve is arranged with a mechanical, key-operated lock for controlling and locking the valve in a selected position. When the valve is in a "locked" position it allows the brakes to be set and the vehicle maintained in situ to prevent the unauthorized movement thereof. The brake assemblies are also secured and locked to prevent the manual operation thereof by use of a conventional lockout tool.