In which cases must you activate the release valve?

The correct answer involves a combination of choices related to the use of the release valve in trailers. The release valve is crucial in specific situations to ensure the safe operation of trailers when air brake systems are involved.

1. Activating the release valve when the air tanks are full and the brake lines are not connected is necessary to move the trailer safely. This ensures that the trailer can be maneuvered without being restrained by the braking system, which might otherwise engage due to pressure.

2. Before setting off, activating the release valve can help fill the air tanks of the trailer, allowing for proper braking function during transit. If the trailer's air tanks are not filled, the brakes may not function adequately, leading to dangerous driving conditions.

3. Additionally, before departing, releasing the parking brake of the trailer is critical. If the parking brake is engaged and the release valve is not activated, the trailer would remain immobilized, posing a risk during maneuvers such as starting to drive or for subsequent connections/disconnections.

Understanding the combination of these actions is vital according to German traffic regulations to guarantee both the safety and functionality of the vehicle and trailer system on the road.