When is it appropriate to use the hand valve to park a combination vehicle?

Using the hand valve to park a combination vehicle is never appropriate, as it can lead to serious safety hazards. The hand valve is designed for other purposes, such as testing the trailer's air supply or manipulating the trailer brakes while the vehicle is at a complete stop. Engaging the hand valve while the vehicle is parked could inadvertently release the brakes if the system is not properly set, risking the vehicle rolling away. Proper parking procedures involve using the parking brakes, which are designed to secure the vehicle in place safely. Thus, it is crucial to understand that the hand valve should not be used for parking purposes in any situation.