Air Brakes · Section 5.1

Air brake system parts: CDL practice questions

Section 5.1 of the CDL manual names the parts of an air brake system and what each one does: the compressor and governor, the air tanks and drains, the gauges and warning devices, the spring brakes and the brake controls. It is the largest part of the Air Brakes test.

Examlane has 79 questions on this chapter. Ten are below; open one to see the answer and why. The rest are in the practice tests.

Practice this chapter

1. What does the air compressor governor decide?
1. When the brake lights come on
2. How much air the brake pedal lets out
3. When the compressor pumps air into the tanks
4. When water drains from the tanks
Answer: C. When the compressor pumps air into the tanks
The governor controls when the compressor pumps air into the storage tanks.
Based on: CDL manual, Section 5.1.2 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
2. About what tank pressure makes the governor stop the compressor from pumping air?
1. 55 psi
2. 100 psi
3. 150 psi
4. 125 psi
Answer: D. 125 psi
The governor cuts out at around 125 psi. It lets the compressor pump again when pressure falls to about 100 psi.
Based on: CDL manual, Section 5.1.2 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
3. The air compressor stops working. What can the air storage tanks still do?
1. Hold enough air for several brake applications
2. Hold air for only one brake application
3. Keep the brakes working normally for the rest of the trip
4. Pump air back into the compressor
Answer: A. Hold enough air for several brake applications
The tanks hold enough air to let you use the brakes several times even if the compressor stops, but the air will run out.
Based on: CDL manual, Section 5.1.3 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
4. Why is water in the compressed air a danger in cold weather?
1. It makes the governor cut out too early
2. It can freeze and cause brake failure
3. It makes the compressor pump faster
4. It makes the brake linings wear faster
Answer: B. It can freeze and cause brake failure
Water in the air system can freeze in cold weather, and frozen parts can make the brakes fail.
Based on: CDL manual, Section 5.1.4 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
5. Where do water and oil tend to collect in an air tank?
1. At the top of the tank
2. Near the safety valve
3. Near the gauge connection
4. At the bottom of the tank
Answer: D. At the bottom of the tank
Water and oil settle at the bottom of the tank, which is why the drain valve is placed there.
Based on: CDL manual, Section 5.1.4 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
6. When must you drain an air tank that has a manual drain valve?
1. Once a week
2. Once a month
3. When you finish driving each day
4. Only when the low air warning comes on
Answer: C. When you finish driving each day
Manual drain valves must be opened by you at the end of every day of driving to remove water and oil.
Based on: CDL manual, Section 5.1.4 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
7. What helps keep the automatic drain of an air tank from freezing in cold weather?
1. A heating device
2. A longer drain cable
3. A larger air tank
4. Extra compressor oil
Answer: A. A heating device
Automatic air tanks can have heating devices that help prevent the automatic drain from freezing.
Based on: CDL manual, Section 5.1.4 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
8. An air brake system has an alcohol evaporator. What is it for?
1. To clean oil out of the air tanks
2. To cool the brake drums on long hills
3. To lower the risk of ice in the valves and other parts
4. To raise the pressure at which the governor cuts out
Answer: C. To lower the risk of ice in the valves and other parts
The evaporator puts alcohol into the air system to reduce the chance of ice in cold weather, because ice can stop the brakes from working.
Based on: CDL manual, Section 5.1.5 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
9. At about what pressure is an air brake safety valve usually set to open?
1. 55 psi
2. 125 psi
3. 150 psi
4. 100 psi
Answer: C. 150 psi
The safety valve usually opens at 150 psi to protect the tank and the system from too much pressure.
Based on: CDL manual, Section 5.1.6 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
10. You find that the safety valve has released air. What is the right response?
1. Treat it as normal and ignore it
2. Have a mechanic fix the problem
3. Plug the valve to hold the air in
4. Drain the tanks and keep driving
Answer: B. Have a mechanic fix the problem
A safety valve that releases air means something is wrong, so a mechanic should fix the fault.
Based on: CDL manual, Section 5.1.6 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.

Read the chapter itself in your state's CDL manual, Section 5.1. Your state's manual is free: find it in the state table.

More Air Brakes chapters: Inspecting air brakes.