When should the trailer service brakes be applied?

The appropriate time to apply the trailer service brakes is when the air pressure is normal. This ensures that the brake system is fully operational and capable of providing the necessary stopping power. In a vehicle equipped with air brakes, maintaining proper air pressure is crucial for the brakes to function correctly. When the air pressure is within the recommended range, it indicates that the brake system is charged and ready to engage, allowing for effective slowing or stopping of the trailer.

When considering the other options, engaging the parking brakes typically occurs when the vehicle is parked or at a complete stop, while applying service brakes while the trailer is still refers to stopping the trailer before initiating movement. The service brakes should be engaged in response to traffic conditions or when stopping the vehicle, rather than solely based on the trailer's motion.