What should be checked regularly to ensure proper function of the air brake system?

Answer: C — Air pressure levels

To ensure the proper function of the air brake system, it is essential to regularly check air pressure levels. The air brake system operates by using compressed air to engage the brakes, meaning that adequate air pressure is critical for the system to function correctly. If the air pressure drops too low, the brakes may not engage properly, leading to decreased braking efficiency and increased risk of accidents. Monitoring air pressure levels helps ensure that the compressor is functioning correctly and that there are no leaks in the system. Proper air pressure is imperative for the safe operation of commercial vehicles equipped with air brakes.