In what scenario is it essential to use the hand valve?

The hand valve is particularly important when braking on slippery surfaces due to its ability to give the driver greater control over the braking force applied to the trailer. By using the hand valve, the driver can engage the trailer brakes independently from the vehicle's service brakes. This allows for a more gradual and controlled braking action, which is crucial in slippery conditions since applying full brakes can lead to skidding or loss of control.

When driving on slick or icy roads, it's vital to manage the braking force carefully to prevent the wheels from locking up, as this could cause a dangerous situation. The hand valve enables the driver to adjust the braking to the conditions, ensuring a safer maneuver compared to relying solely on the service brakes. In more routine driving, parking, or going downhill, the standard braking systems are generally sufficient and more appropriate for those specific situations.