Abstract: An anti-theft device is coupled in series with a supply line and a service line of a trailer. When activated, the anti-theft device prevents theft of the trailer by decoupling the supply and service lines of the trailer from the reservoir and brake cylinders of the trailer, and by coupling the supply and service lines to the atmosphere, thereby preventing a prospective thief from releasing the trailer's brakes by restoring air to the supply and service lines. When the anti-theft device is deactivated, the supply and service lines are re-coupled to the reservoir and brake cylinders to allow the driver to release the trailer brakes.