When driving a set of doubles, which shut-off valve in the last trailer must be closed?

When driving a set of doubles, closing the shut-off valve in the last trailer is essential for safety and proper operation. The back shut-off valve of the last trailer should be closed to prevent any potential leakage of air from the braking system. This ensures that the brakes on the last trailer are operational and prevents the possibility of air loss during travel, which could lead to the trailer becoming uncontrollable.

If the back valve were left open, it could cause issues such as inadequate air pressure, making it difficult to stop effectively or risking that the brakes will not engage. Thus, ensuring that the back shut-off valve is correctly closed helps maintain the integrity of the braking system throughout the entire combination vehicle, promoting safer driving conditions.