Final answer:
The hand valve should be used to test the trailer brakes, ensuring they are working correctly before a trip. It is not intended as a parking brake and should not be used with the foot brake due to potential control issues.
Step-by-step explanation:
The hand valve, also known as the trolley valve or Johnson bar, should be used to test the trailer brakes. It should not be used as a parking brake because it applies the trailer brakes only and could lead to a dangerous situation if the trailer moves unexpectedly.
Using the hand valve in conjunction with the service brake (foot brake) can lead to uneven braking and potential loss of control. The proper procedure is to use the hand valve independently to ensure that the trailer brakes are working correctly before starting a trip. Each state may have specific regulations, and it's important for drivers to follow the guidelines and training provided by their employer or the jurisdiction in which they are operating.
One of its functions is to be used as a parking brake. When engaged, it locks the wheels in place and prevents the vehicle from moving. It can also be used to test the trailer brakes by applying the hand valve while observing the braking action of the trailer.