What would a driver primarily check when testing the trailer service brakes?

When testing the trailer service brakes, the primary focus is on brake response. This involves ensuring that the brakes engage and function as intended when the driver applies them. Testing for brake response means checking if the brakes stop the trailer smoothly and effectively, which is critical for safety while driving.

A proper brake response indicates that the braking system is working correctly and that the trailer will stop as expected when the brakes are engaged. This is essential for preventing accidents and maintaining control over the vehicle, especially when navigating turns or when the trailer is loaded.

While the other options, such as checking fluid levels, trailer sway, and tire pressure, are also important for overall vehicle safety and operation, they do not specifically evaluate the performance of the trailer service brakes. For instance, fluid levels are crucial for ensuring that brakes are operational, but they do not measure how well the brakes respond when applied. Similarly, checking for tire pressure is important for maintaining traction and handling, but it does not assess the braking ability itself. Trailer sway is a concern when driving but does not directly relate to the effectiveness of the brakes at stopping the trailer.