Abstract: A method and apparatus is provided for securing trailer doors against tampering. In one exemplary embodiment, a locking device is mounted on the inside of a trailer door. The device includes a locking bolt which is selectively operated by a pneumatic valve in response to a signal selectively generated by an operator. In the example, the locking device is normally in a locked position and the locking bolt is effective to secure the trailer doors to the frame of the trailer. An operator selectively causes the generation of an "unlock" signal through the use of a wireless control device. The unlock signal, in turn, operates upon an electro-pneumatic valve device to effect an application of locally available high air pressure to a locking bolt actuator assembly. The air pressure is sufficiently high to effect the movement of a relatively heavy locking bolt out of a locking position, at which time the trailer doors may be opened for access to the trailer load.