Trailer coupling for a motor vehicle

The coupling (1) has a ball neck bearing head (3) attached to a body of a motor vehicle. A control mechanism (7) is arranged into the bearing head. An actuating system is mediated by a hydraulic device. A locking pin (8) determines that the locking bolt is axially actuated and partially extended into a hydraulic medium. The hydraulic medium is provided in a hydraulic chamber. A retractable hydraulic piston controls the locking pin by the hydraulic chamber. Another retractable hydraulic piston and the hydraulic medium are arranged in the former hydraulic piston.