Abstract: An anti-theft device is installed in a vehicle brake line which carries braking fluid. The device defines a valve seat and a valve body adapted to close against said valve seat, the valve body having a passageway therethrough and being rotatable against said seat about an axis substantially perpendicular to the passageway, between a first position in which it permits fluid flow in one direction and inhibits fluid flow in the opposite direction, and a second position in which it permits fluid flow in both directions. The valve can easily be switched between the first and second positions, and can be locked in the first position. By allowing braking fluid to flow only toward the wheels, it will be evident that the wheel brakes, once applied, cannot, be released.