In testing the trailer brakes, what should occur when the spring parking brake is released?

Answer: C — The trailer service brakes will not function

When the spring parking brake is released, the proper response is that the trailer service brakes will function. This is because the service brakes are activated when air pressure is applied, which typically occurs when the parking brake is released. The spring parking brake is designed to engage and hold the trailer's brakes in place when the vehicle is stationary, but when released, it allows the system to function normally. If the spring parking brake is properly working, releasing it should enable air to flow into the system, thus allowing the trailer service brakes to engage when the foot pedal is pressed. Therefore, stating that the trailer service brakes will not function contradicts the correct operation of the braking system under normal conditions. The expectation is that service brakes are operational when the parking brake is disengaged, reflecting a potentially unsafe condition if they're not functioning as designed.