How should you test the trailer service brakes?

Testing the trailer service brakes using the hand control, often referred to as the trolley valve, is essential because it allows you to assess the function of the trailer brakes independently from the tractor's braking system. This method involves applying pressure to the hand control to engage the trailer brakes while the vehicle is stationary.

This test helps to ensure that the trailer's brakes are functioning correctly and provides immediate feedback on their responsiveness. It also allows you to identify any potential issues with the brakes, such as dragging or non-functioning brake components, which could be dangerous when driving on the road.

Using the parking brake or simply pressing the brake pedal firmly does not accurately assess the performance of the trailer's service brakes separately. Checking for leaks in the brake line, while important for overall brake system integrity, does not assess the efficiency or operation of the trailer's service brakes directly. Therefore, using the hand control effectively evaluates the braking system's readiness and performance, making it the most appropriate method for testing trailer service brakes.