Overrun brake

An overrun brake—also called an inertia brake or surge brake (in North America for hydraulic versions)—is a brake system that allows a trailer or caravan to brake proportionally to the deceleration of its towing vehicle. The system uses the trailer’s own momentum acting through the coupling to operate its brakes automatically, without a separate electrical or pneumatic control line.

Overrun braking has been widely used in light trailers across Europe, Australia, and New Zealand since the mid-twentieth century, where full air-brake systems are impractical on small trailers. In the United States, surge-type hydraulic actuators serve a similar function for boat and utility trailers.