Abstract: An antitheft device is provided for a vehicle such as a tractor/trailer combination, with an engine operable to develop pneumatic pressure in a reservoir during operation of the engine, and a pneumatic conduit leading from the reservoir to an operational load, preferably a brake release mechanism. The antitheft device preferably has at least two controllable valve along the pneumatic conduit, each with at least two positions including a first position at which the pneumatic conduit is open through the valve from the reservoir to the operational load, and a second position at which the fluid conduit is closed between the reservoir and the load by the valve means to disable the operational load. Air pressure is diverted at the valve to a branch coupling the reservoir through to an alarm such as an air horn. The antitheft device can be included in a tractor or a trailer, and preferably valves and alarms are provided in both.