Abstract: A system for installation in mechanized vehicles which have an hydraulic brake system prevents the unauthorized operation of such vehicles and includes a valve of a special construction which comprises a housing provided with inlet ports and outlet ports for the hydraulic fluid, and an inner core member which is movable within the housing and is provided with bores which correspond with the ports in the housing forming a passage. Further, the valve includes at least one by-pass passage provided with a one way valve, the inner core member is movable rotatively or axially within the housing, and is connected to a key operated lock.