Abstract: An anti-theft device for protecting air brake control valves from being manipulated is disclosed. A clamp assembly locks on to the valve knobs employed to release the air brakes in a vehicle. A pair of blocks incorporate a key lock assembly and posts to enable portable use of the device. This device is readily adaptable to lock more than one type of several standard brake valve knob sizes by utilizing removable inserts. Impact-resistant plastic or metallic compositions may form the bodies.