"Dead end" couplers should be used on air lines that are not in use to prevent what?

Using "dead end" couplers on air lines that are not in use is primarily intended to prevent contamination. When air lines are left open or unprotected, they can easily draw in dirt, moisture, and debris from the environment. This contamination can not only affect the air quality in the brake system but also potentially lead to malfunction or failure of the braking components that rely on pneumatic pressure.

By sealing these air lines with dead end couplers, you effectively block the entrance of contaminants, ensuring that the air delivered to the air brake system remains clean and functional. This practice is essential for maintaining the integrity and performance of the air brake system over time.