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

You answered correctly. The braking action of a trailer with overrun brakes starts when the trailer runs up to the towing vehicle. This happens because overrun brakes are mechanical and are activated by the force of the trailer pushing against the vehicle as it slows down. The brake pedal in the towing vehicle does not directly engage the trailer's brakes. This design helps slow the trailer down automatically, especially on downhill slopes or when reducing speed, making it an important safety feature for trailers without a direct braking connection. Remember: for trailers with overrun brakes, the braking is initiated by the relative movement between the trailer and the towing vehicle, not by pressing the truck's brake pedal itself.