Glad hand lock is the perfect device to dead-line a trailer, offering a simple way to prevent the trailer from being moved while being loaded or unloaded. This semi-trailer lockout device acts as a harness around the air intake valve of the emergency brake system. The trailer is immobilized because the glad hand lock must be removed before the air line can be connected, thereby releasing the trailer emergency brake. Lock is made of heavy duty corrosion and rust resistant cast iron that can be installed in seconds. Cylinder lock prevents removal without the key and dock supervisors can assign keys to loads to certify that the proper driver is taking the correct load. Keyed Alike for easy trailer security.