Parts of an air brake system, explained
Section 5.1 of the Kentucky CDL Manual (p. 5-1) names each part of an air brake system and what it does. It is the largest part of the Air Brakes test in our bank. It is tested on the Air Brakes test.
Key facts
| Part of | the Air Brakes test (80% to pass) | KY CDL Manual, front section pp. 2-3 |
|---|---|---|
| Manual section | §5.1 Parts of an air brake system | KY CDL Manual p. 5-1 |
| Questions in CDL Mile's bank | 41 | CDL Mile, checked against KY CDL Manual |
Key rules
- § 5.1.5p. 5-2
An alcohol evaporator adds alcohol to the air system. This cuts the chance of ice forming in brake valves and other parts in cold weather, and ice in the system can stop the brakes from working.
Alcohol Evaporator
- § 5.1.5p. 5-2
In cold weather, check the alcohol container and fill it as needed every day. The evaporator does not get rid of water and oil, so tanks without automatic drains still need draining every day.
Alcohol Evaporator
- § 5.1.2p. 5-1
The governor's cut-in pressure is around 100 psi. Once tank pressure falls to that level, the governor lets the compressor start pumping again.
Air Compressor Governor
- § 5.1.8p. 5-2
Foundation brakes are used at each wheel, and the s-cam drum brake is the most common type. Wedge brakes and disc brakes are less common.
Foundation Brakes
- § 5.1.2p. 5-1
About 125 psi is the cut-out level. When tank pressure reaches it, the governor stops the compressor from pumping more air into the tanks.
Air Compressor Governor
Try two questions
Air Brakes · Parts of an air brake system
The safety relief valve on an air brake system is usually set to open at about what pressure?
Tap an answer to see the explanation and the manual page.
Air Brakes · Parts of an air brake system
While building air pressure, you hear the safety valve on the first air tank letting air out. What should you do?
Tap an answer to see the explanation and the manual page.
More Air Brakes topics
Sources
Kentucky Commercial Driver's License Manual, version March 2025, Kentucky State Police. Page references are to its printed page numbers. CDL Mile is independent and not affiliated with Kentucky State Police.
Updated for the March 2025 Kentucky CDL manual. Last checked November 5, 2026.
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