In which cases must you activate the release valve?

You must activate the release valve when the air tanks are full, the brake lines are not connected, and the trailer needs to be moved. This is because, without the brake lines connected, the trailer’s braking system remains engaged by default to prevent rolling away. Activating the release valve temporarily releases the trailer's parking brake, allowing you to move the trailer safely without air pressure from the towing vehicle. The other options relate to preparing the trailer for driving or releasing brakes after connection, but do not apply if the brake lines are disconnected and the trailer must be moved. Understanding this helps ensure safe handling of trailers in different connection states.