Why should you lock the tractor's glad hands to each other when not towing a trailer?

Locking the tractor's glad hands to each other when not towing a trailer is essential primarily to keep dirt and water out of the lines. Glad hands are the connectors for air lines between the tractor and trailer, and when disconnected, they can be exposed to environmental elements. Keeping them locked together prevents contaminants from entering the air brake system, which can lead to malfunction and serious safety issues when the vehicle is in operation. This practice helps ensure that the air lines remain clean and functional, promoting the reliability of the braking system when the tractor is eventually used with a trailer.