Illinois CDL Practice Exam

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Prepare for the Illinois CDL Exam. Study with flashcards and multiple-choice questions, complete with hints and explanations. Boost your confidence and get ready to pass your CDL exam!

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What is the purpose of the Trailer Hand Valve or Johnson bar?

To control the speed of the trailer on downhill slopes
To activate the trailer lights during night driving
To operate the trailer brakes for testing purposes only
To adjust the trailer suspension height

The correct answer is: To operate the trailer brakes for testing purposes only

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.