When should you use the hand valve to park a combination vehicle?

Using the hand valve to park a combination vehicle is not recommended because it can lead to unsafe situations. The hand valve, which controls the trailer's brakes, can disengage upon parking if the vehicle shifts or settles, potentially causing the vehicle to roll if not properly secured. This is particularly dangerous on a slope or when loading and unloading at docks, where stability is essential. Instead, the parking brakes should be utilized, which are specifically designed for securing the vehicle when parked, ensuring all wheels are reliably held in place. Understanding the proper methods for securing both the cab and trailer is crucial for the safety of the driver and the integrity of cargo.