Illinois CDL Practice Exam 2024 - Free CDL Practice Questions and Study Guide.

The Trailer Hand Valve or Johnson bar is used to operate the trailer brakes for testing purposes only. This valve allows the driver to apply the trailer brakes independently of the tractor brakes, which can be useful when conducting brake checks or when performing maintenance on the braking system. It is important for drivers to be familiar with the functions of all the controls in the vehicle to ensure safety on the road. Option A is incorrect because controlling the speed of the trailer on downhill slopes is typically done using the service brakes and engine retarder, not the Trailer Hand Valve. Option B is incorrect because activating the trailer lights during night driving is usually done using the light controls on the dashboard, not the Trailer Hand Valve. Option D is incorrect because adjusting the trailer suspension height is usually done using dedicated controls within the suspension system, not the Trailer Hand Valve.