Practice Exam Questions And Correct

Illinois CDL Air Brakes Endorsement Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the purpose of the air brake system on a commercial vehicle?
• To increase fuel efficiency
• To stop and hold the vehicle safely
• To control engine speed
• To reduce tire wear

Air brakes use compressed air to apply force to the brake shoes or pads, ensuring safe stopping and holding of the vehicle.

• What is the main advantage of air brakes over hydraulic brakes?
• Simpler design
• Lower maintenance cost 1 / 4

• Ability to handle heavy loads
• Faster response time

Air brakes can generate more stopping power and are suitable for heavy trucks and trailers, unlike hydraulic brakes.

• How often should you check the air pressure in your vehicle’s brake

system?

• Once a week
• Once a month
• Before each trip
• Every 500 miles

Checking air pressure before every trip ensures that brakes function correctly and prevents accidents.

• At what air pressure should the low-pressure warning signal activate in

a dual air brake system?

• 25 psi
• 50 psi
• 70 psi
• 60 psi

The low-pressure warning must activate before air pressure drops below 60 psi to alert the driver to potential brake failure.

• What is the purpose of the air compressor in the air brake system?
• To cool the brakes
• To release air from the system 2 / 4

• To supply air to the storage tanks
• To regulate speed

The air compressor supplies compressed air to the storage tanks, which is essential for brake operation.

• What does the air dryer do in an air brake system?
• Increases air pressure
• Removes moisture and contaminants from the air
• Lubricates the brakes
• Controls braking force

Moisture and contaminants can freeze or corrode components, so the air dryer ensures clean, dry air in the system.

• What should you do if the air pressure drops below 60 psi while

driving?

• Continue driving carefully
• Stop safely and check the system
• Pump the brakes rapidly
• Increase speed

Air pressure below 60 psi can cause brake failure, so you must stop and inspect the system immediately.

• How much air pressure is typically required for full brake application?
• 40–60 psi
• 60–80 psi
• 80–90 psi 3 / 4

• 90–120 psi

Most commercial vehicles operate between 90–120 psi for effective air brake application.

• What happens if the air tanks are not drained regularly?
• Air pressure increases
• Water and oil accumulate, causing brake failure
• Brakes apply automatically
• Nothing happens

Water and oil can reduce brake efficiency or freeze, leading to dangerous situations.

• What is the function of the brake chambers?
• Supply air to the compressor
• Convert air pressure into mechanical force to apply brakes
• Store air
• Regulate speed

Brake chambers push the pushrod when air pressure is applied, moving brake shoes or pads to stop the vehicle.

• How should you test the air brakes during a pre-trip inspection?
• By applying the brakes at high speed
• By checking air pressure buildup and performing a low-pressure

warning test

• By pumping the brakes rapidly
• By listening for leaks only
• / 4