Glad hand lock is the perfect device to dead-line a trailer, offering a simple way to prevent a 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 can be installed and removed 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. Lock must be removed before the air line can be connected, thereby releasing the trailer emergency brake.