Illinois CDL Practice Exam 2025 - Free CDL Practice Questions and Study Guide

The correct answer is that water and oil are typically expelled from air tanks in an air brake system through an automatic method. This functionality is crucial for maintaining the effectiveness and safety of the air brake system. Automatic drainage helps to ensure that moisture and contaminants do not accumulate inside the air tanks, which could lead to corrosion, freezing, or other issues that may affect braking performance.

In modern air brake systems, automatic drain valves are commonly used. These valves are designed to open and release accumulated water and oil without the need for the driver to intervene manually. This is particularly helpful in ensuring that the system remains clear of contaminants during operation, which can be difficult to manage in terms of regular maintenance.

The other methods mentioned, while they might be useful in specific contexts, do not typically pertain to the automatic expulsion of unwanted substances from air tanks. Therefore, the reliance on an automatic method for draining air tanks is a critical aspect of ensuring the continuous and reliable operation of air brake systems.