If air pressure does NOT build up to normal within the correct amount of time, then:

If air pressure does not build up to normal within the specified amount of time, it indicates a potential problem with the air brake system. The primary function of the air compressor is to build up and maintain adequate air pressure in the system for proper operation of the brakes. When the pressure is insufficient, it may lead to inadequate braking capability while driving.

This situation can pose serious safety risks; if the air pressure drops too low during operation, the braking system may not respond effectively when needed, requiring an emergency stop. This scenario underscores the critical nature of maintaining proper air pressure, as it ensures that the driver can achieve effective deceleration and stop the vehicle safely.

The other options do not address the core issue of inadequate air pressure leading to potential brake failure or a compromised stopping ability while the vehicle is in motion. Thus, ensuring that air pressure builds up adequately is crucial for the safe operation of air brake systems in heavy vehicles.