CDL AIR BRAKE Study Guide 2026

The supply pressure gauge shows the driver the current air pressure in the tanks so they know if adequate braking power is available.

If the low air pressure warning activates, it indicates a critical issue with the air brake system, meaning there might not be enough air to safely operate the brakes. Continuing to drive risks complete brake failure, so the safest and most responsible action is to immediately and safely stop the vehicle to prevent an accident.

During a pre-trip inspection, it's crucial to thoroughly check the air compressor. This includes verifying that it is securely mounted, free from any visible damage or leaks, and operating correctly. You must confirm that it builds and maintains air pressure within the specified range by turning on and off automatically, ensuring the entire air brake system is functional.

The primary purpose of ABS (Anti-lock Braking System) is to prevent the wheels from locking up during hard braking. This allows the driver to maintain steering control and vehicle stability, especially on slippery surfaces. While it can sometimes reduce stopping distances, its main function is to prevent uncontrolled skidding and improve control.

If the ABS warning light stays on, it indicates a malfunction in the Anti-lock Braking System. While the ABS itself may not be working, the vehicle's regular service brakes will still function normally, allowing you to stop. It is important to get the ABS checked by a mechanic as soon as possible to restore this important safety feature, but it does not require an immediate stop or tow.

A modulating control valve allows the driver to control the application of the spring brakes, typically through a lever on the dash. This provides a 'feel' for the braking action and allows for a controlled stop if the service brakes fail.