What is the effect on a truck-trailer combination when the parking brake in the towing vehicle is engaged (not in the test position)?

When you engage the parking brake in the towing vehicle of a truck-trailer combination, the towing vehicle itself is mechanically braked. This is because applying the parking brake activates the mechanical brake on the towing vehicle. At the same time, the trailer is generally only braked via the compressed air service brake system, not by a mechanical parking brake. This means the trailer does not receive a mechanical (spring-type) brake application unless there is a total loss of air. Therefore, selecting both the towing vehicle is mechanically braked and the trailer is usually only braked with compressed air is correct. This rule is important to remember for proper handling and parking of truck-trailer combinations, ensuring that you know how each part of the vehicle is secured when the parking brake is used.