Final answer:
Air tanks in air brake systems should be drained daily to prevent water and oil buildup, which can lead to brake failure.
Step-by-step explanation:
The question pertains to the maintenance of the air tanks used in air brake systems, commonly found on larger vehicles such as trucks and buses. The correct frequency for draining the air tanks is daily. Water and oil buildup in the air tanks can cause brake failure or other issues. Therefore, to ensure the proper functioning and safety of the air brake system, the air tanks should be drained at the end of each day of vehicle operation.