ASE School Bus Technician Brakes Certification (S4) Practice

The function of an air drier in an air brake system is to capture water and oil using a desiccant material. This is crucial because moisture in the air system can lead to corrosion, freeze-ups, and overall inefficiency in the braking system. The drier removes contaminants from the compressed air before it reaches the brake components, ensuring that the air remains dry and clean.

In the operation of an air drier, air passes through the drier unit where it encounters the desiccant. The desiccant absorbs moisture from the air, which is vital in maintaining the integrity and reliability of the braking system. This process helps prevent issues that could arise from excessive moisture, such as compromised performance or damage to air-operated components.

Other options refer to functions that are not characteristic of an air drier's purpose. For example, warming air prior to compression is typically a function of a heater, while lubricating the air system is handled by separate components designed specifically for that purpose. Controlling airflow direction may occur within the air system but is not a function of the air drier itself. Thus, the accurate role of the air drier is specifically to eliminate moisture and oil, maintaining the efficiency and safety of the brake system.