Overrun brake

Later systems were fitted with a sliding mechanism within the coupling, which enables the drawbar to move back and forth relative to the trailer chassis.

The sliding mechanism contains a damper to even out the shock loading transmitted from the tow vehicle.

Some systems have a ball hitch that is normally straight, but when the lead vehicle starts braking, the trailer pushes forward on the ball hitch, pitching it up which then activates a variable hydraulic brake system.

Upon activating the trailer slows down pulling back on the hitch again, making it level then that shuts off the braking system.

Because of its issues, all OECD countries limit the maximum road legal towable weight (UK up to 3500kg or 7716lbs) using such a braking system.