Final answer:
The hand valve should be used in specific situations such as parking at loading docks, short-term parking, and testing trailer brakes.
Step-by-step explanation:
The hand valve should only be used when you park at loading docks, when you are parking for less than one hour, and to test the trailer brakes.
When parking at a loading dock, the hand valve is used to prevent the trailer from moving while it is being loaded or unloaded. This ensures safety and stability while the trailer is stationary.
When parking for less than one hour, using the hand valve can prevent the trailer from rolling or moving unintentionally. It adds an extra layer of security during short stops.
To test the trailer brakes, the hand valve is applied while the truck is in motion. This allows the driver to check if the brakes on the trailer are functioning properly before a trip.