What should be checked for the main air line?

Answer: C — Connections should be tight without any leaks

The main air line is a critical component of a commercial vehicle's air brake system, and ensuring its integrity is essential for safe operation. The correct answer emphasizes that connections should be tight and free from leaks. Tight connections are vital because any air leaks can compromise the braking system's effectiveness and lead to potential failures while on the road. A secure main air line helps maintain proper air pressure levels necessary for the operation of air brakes, ensuring the vehicle can stop safely and effectively when needed. Maintaining a leak-free and properly connected air line is crucial in preventing unnecessary air loss which can affect the overall performance of the braking system. Regular checks of the air lines for tightness and leaks should be part of the pre-trip inspection process to ensure the vehicle is in proper working condition before it is driven.