Which action is part of the pre-trip inspection for air pressure?

During a pre-trip inspection, it is crucial to check the air pressure in the air brake system to ensure it is sufficient for safe operation. The correct action is that the air pressure must be building. This means the air compressor is functioning correctly and the system is reaching the appropriate pressure needed for the brakes to operate effectively.

When the air pressure is building, it indicates that the compressor is working to replenish the air supply after it has been used, ensuring there is enough air available for braking when needed. Insufficient air pressure may lead to brake failure or suboptimal braking performance, posing a serious safety risk.

The other potential answers reflect circumstances that would not align with safe operations: a decreasing air pressure indicates a leak or malfunction in the system; zero air pressure suggests that the air pressure has been completely depleted, which would mean the brake system cannot function; and the absence of air pressure altogether implies an inoperative air brake system. Therefore, confirming that the air pressure must be building is a critical step in ensuring the brakes are ready for operation.