Parts of an air brake system, explained

Section 5.1 of the CDL handbook 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.

Key facts

§ 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

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 handbook 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 handbook page.

Texas Commercial Motor Vehicle Drivers Handbook (DL-7C), revised March 2026, Texas Department of Public Safety. Page references are to its printed page numbers. CDL Mile is independent and not affiliated with Texas DPS.

Updated for the March 2026 Texas CDL handbook (DL-7C). Last checked November 4, 2026.

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