Illinois CDL Air Brakes Practice Test 2026

Illinois Air Brakes CDL Practice Test

1. If your vehicle doesn't have automatic tank drains, you should drain the oil and water from the bottom of the air tanks ___.

   • Before starting the engine each morning.
   • At the end of each day of driving.
   • Every month.

   Drain the oil and water from your truck's air tanks at the end of each day of driving if they don't have auto drains. This keeps the brakes working right.

2. What does the air compressor governor control?

   • The amount of fuel the engine uses
   • When air is pumped into the air tanks.
   • The temperature of the engine.

   The air compressor governor decides when to fill up the air tanks used for brakes and other parts. It doesn't control the engine's fuel use or temperature.

3. If you pull hard on a slack adjuster and it moves more than ___, it needs to be adjusted.

   • 1 inch
   • 5/8 inch
   • 3 inches

   If the slack adjuster moves more than 1 inch when you pull on it, it's too loose and needs adjusting. This is important for the brakes to work right.

4. If the air pressure doesn't go up to the needed level fast enough, then:

   • The headlights could become dimmer.
   • You might need to adjust the mirrors more frequently.
   • Air pressure may drop to a low point while driving, requiring an emergency stop.

   If the air pressure in your truck doesn't build up fast enough, it can get too low while you're driving. This means you might have to suddenly stop your truck to avoid problems.

5. Parking or emergency brakes must be kept in position by something that does not leak, like:

   • Spring pressure.
   • Magnetic force
   • Electric current

   The right answer is spring pressure. This is because parking or emergency brakes in vehicles need a reliable way to stay active. Springs can do this without the risk of losing power, unlike systems that depend on magnets or electricity, which can fail if there's a power loss or leak.

6. Excessive heat caused by overusing your brakes can also cause ___.

   • Tire pressure to decrease.
   • Brake to fade or fail.
   • Engine oil to overheat.

   Using your brakes too much can make them too hot. When they get too hot, they might not work right or could stop working altogether.

7. At what psi does the air compressor stop pumping air?

   • 135
   • 125
   • 140

   The air compressor stops pumping air at 125 psi because this is the standard pressure level needed to maintain the air brake system's functionality without overloading it. This ensures that the system has sufficient air pressure to operate safely while preventing potential damage or inefficiencies from excessive pressure.

8. What brake system applies and releases the brakes when the driver steps on the brake pedal?

   • Manual
   • Service
   • Automatic

   The correct answer is "Service." This is the brake system in a vehicle that works when the driver pushes the brake pedal to slow down or stop the truck. It's the main system used for everyday driving.

9. The safety valve lowers pressure when it reaches ___ psi.

   • 150
   • 100
   • 50

   The safety valve helps keep the air pressure in the brakes from getting too high. It does this by letting some air out when the pressure hits 150 psi. This makes sure the brakes work right and stay safe.

10. What happens if your vehicle's brake drums overheat?

    • They expand.
    • They crack and leak fluid.
    • They glow bright red

    When your brake drums get very hot, they expand. This means they get bigger because of the heat.

11. Why is it important to remove water from the compressed air tanks?

    • Water can freeze in cold weather and cause brake failure.
    • Water is removed because its presence might cause the tank to change color, which is thought to be a sign of wear.
    • Water is removed from the compressed air tanks to make the system lighter and improve fuel efficiency.

    Draining water from air tanks is important because in cold weather, it can freeze and stop your brakes from working right.

12. When going down a steep hill and reaching your safe speed of 40 mph, you would use the brake until your speed goes down to _____ mph.

    • 35
    • 30
    • 25

    When driving down a steep hill and you're at your safe speed of 40 mph, hit the brakes until you slow down to 35 mph. Then, let go of the brakes to avoid overusing them.

13. When you use the service brakes too much, they can get too hot and this can cause ___.

    • Decreased fuel efficiency
    • Reduced tire pressure
    • Expansion of the brake drums.

    When you use the brakes too much, they can get really hot. This heat can make the brake drums (part of the brakes) get bigger. This doesn't help the car save gas or change the air in the tires; it just makes the brakes work less effectively.

14. Your truck is equipped with a dual air brake system and minimum-size air tanks. How long should it take for the air pressure to go up from 85 to 100 psi?

    • 30
    • 45
    • 60

    When your truck's air brake system is in good shape and the tanks are the smallest allowed, the air pressure should go from 85 to 100 psi in 45 seconds or less. This shows your brakes are working right and can safely stop the truck.

15. What does the term "Wig Wag" mean?

    • A mechanical arm that signals low air pressure
    • A lever that controls the speed of the engine.
    • A light that flashes when the engine is overheating

    The term "Wig Wag" refers to a safety device in air brake systems that uses a mechanical arm to alert the driver when air pressure drops to a dangerously low level. This visual warning helps ensure the driver takes immediate action to prevent brake failure, maintaining safety on the road.

16. What statement about spring brake braking power is correct?

    • It depends on the service brakes being in adjustment.
    • It is most effective when the vehicle is fully loaded.
    • It decreases as the vehicle's speed increases.

    The spring brakes on a vehicle work properly only if the regular brakes (service brakes) are set up right. If the service brakes are not adjusted correctly, the spring brakes won't be able to do their job well. So, the power of the spring brakes relies on the service brakes being properly adjusted.

17. How do you normally engage the parking brakes?

    • Apply brakes by pulling the parking brake control knob out.
    • Push the gas pedal down.
    • Shift into neutral gear.

    Engaging the parking brake is all about ensuring your car stays put once parked. By pulling out the control knob, you directly activate the brake system that keeps the vehicle stationary.

18. When the air pressure falls below ___ psi, the straight truck spring brakes will come fully on.

    • 20
    • 60
    • 100

    When driving a straight truck, if the air pressure gets too low, special brakes called spring brakes kick in to stop you. This happens when the air pressure drops below 20 psi (pounds per square inch). It's a safety feature to prevent accidents when there's not enough air pressure for the regular brakes.

19. What is the purpose of a brake light switch?

    • It turns on the brake lights when you put on the air brakes.
    • It adjusts the brightness of your headlights.
    • It activates the turn signals when you apply the brakes

    A stop light switch makes the brake lights turn on when you use the air brakes. This lets other drivers know you're slowing down or stopping.

20. Which of the following types of foundation brakes are most commonly found on heavy vehicles?

    • S-cam drum.
    • Hydraulic disc
    • Air pad

    The S-cam drum brake is the most common brake on big vehicles. It uses a special cam to push brake pads against a drum to stop the vehicle. This system is reliable and widely used in trucks and buses.