Final answer:
The correct answer is option 3: you should never use the hand valve to park a combination vehicle. Instead, the parking brakes should be used for secure and stable parking.
Step-by-step explanation:
The correct answer is option 3: never. The hand valve, also known as the trolley valve or Johnson bar, should not be used to park a combination vehicle. It is intended for the manual application of the trailer brakes only, and using it for parking could result in the loss of control if the trailer brakes are applied too suddenly or unevenly.
Throughout the transportation industry, it's widely taught that the proper method to park a combination vehicle is to use the parking brakes, which engage the brakes on both the tractor and trailer units.
This ensures a more secure and stable parking, especially on grades or at loading docks. Additionally, regulations often require the use of wheel chocks or other safety measures when parking on a grade to prevent the vehicle from rolling.
The correct answer is option 2. You should use the hand valve to park a combination vehicle on a grade. When parking on a hill or an incline, it is important to engage the hand valve in addition to setting the parking brake to prevent the vehicle from rolling.
The hand valve controls the air pressure to the trailer brakes, allowing you to lock them in position and keep the vehicle stationary.