When does the braking action of a trailer with overrun brakes start?

When a trailer is equipped with overrun brakes, the braking action begins when the trailer pushes against the towing vehicle during deceleration—this is described as "when the trailer runs up to the truck." In practical terms, as the towing vehicle slows down, the momentum of the trailer causes it to move forward slightly, compressing a mechanism that activates its brakes. This system ensures the trailer brakes in sync with the towing vehicle, providing smoother and safer braking. It does not start simply when the brake pedal is pressed, nor does it begin before the truck’s brakes to keep the combination extended; instead, it reacts to the physical movement of the trailer relative to the towing vehicle. Understanding this helps in recognizing how trailer brakes reduce stress on the truck and improve overall braking performance.