CDL Air Brakes Knowledge Test — Questions and Answers

Question 1: When should you use parking brakes?

Only when parking in a flat area.
Only when parking on a hill.
During emergency braking situations.
Every time you park the vehicle, unless brakes are hot or in freezing weather. (Correct answer)

Correct answer: Every time you park the vehicle, unless brakes are hot or in freezing weather.

Parking brakes should be used every time you park a vehicle to ensure it remains securely stationary. However, there are exceptions: if the brakes are extremely hot, engaging the parking brake can cause them to warp or fuse. In freezing weather, moisture can freeze the brake components, making them impossible to release, so wheel chocks should be used instead.

Question 2: A driver is operating a combination vehicle and needs to park. What is the correct procedure for using the parking brakes?

Apply the parking brakes for both the tractor and the trailer. (Correct answer)
Use the service brake pedal to hold the vehicle while shutting down.
Apply only the trailer parking brakes to prevent stress on the tractor.
Apply only the tractor parking brake and leave the trailer brakes released.

Correct answer: Apply the parking brakes for both the tractor and the trailer.

When parking any vehicle, especially a combination vehicle, the parking brakes on all units should be applied to secure the entire vehicle safely. The yellow knob applies the tractor parking brakes, and the red knob (trailer air supply) applies the trailer emergency/parking brakes.

Question 3: Why is it important to drain air tanks regularly in an air brake system?

To remove water and oil that can cause brake failure or freeze in cold weather (Correct answer)
To reset the governor cut-in pressure
To allow the compressor to build pressure faster
To increase air pressure in the system

Correct answer: To remove water and oil that can cause brake failure or freeze in cold weather

Water and oil accumulate in air tanks and can lead to brake failure, rust, and freezing in cold temperatures if not drained regularly.

Question 4: How does ABS help in an emergency stop on icy or wet roads?

It increases traction by heating the tires.
It shortens stopping distances regardless of conditions.
It reduces the need to use service brakes.
It prevents skidding and allows you to steer. (Correct answer)

Correct answer: It prevents skidding and allows you to steer.

On icy or wet roads, ABS is crucial because it prevents the wheels from locking up during hard braking. By maintaining wheel rotation, ABS allows the tires to retain some traction, enabling the driver to steer the vehicle and avoid skidding. This significantly improves control and safety in hazardous conditions, rather than simply shortening stopping distances.

Question 5: Which of the following are the three main types of foundation brakes used on vehicles with air brake systems?

Disc, magnetic, and S-cam brakes
Drum, disc, and electromagnetic brakes
S-cam, hydraulic, and wedge brakes
S-cam, disc, and wedge brakes (Correct answer)

Correct answer: S-cam, disc, and wedge brakes

The three common types of foundation brakes are S-cam, disc, and wedge brakes. S-cam brakes are the most common type of drum brake found on heavy commercial vehicles.

Question 6: You are driving a vehicle with a dual air brake system. The low air pressure warning light and buzzer come on. What is the safest course of action?

Stop and safely park the vehicle as soon as possible. (Correct answer)
Downshift and use the engine brake to slow down, preserving the remaining air pressure.
Increase your following distance and avoid using the brakes unless absolutely necessary.
Continue driving to the nearest service station while monitoring the gauges.

Correct answer: Stop and safely park the vehicle as soon as possible.

If the low air pressure warning activates, it indicates a potential air leak or system failure. The safest action is to pull over and stop immediately while you still have enough air pressure to control the service brakes. Continuing to drive is dangerous as the spring brakes could engage automatically when pressure drops too low, causing a sudden and uncontrolled stop.

Question 7: Which air tank typically accumulates the most water and oil contamination?

Primary service tank
Secondary service tank
Trailer reservoir tank
Supply (wet) tank (Correct answer)

Correct answer: Supply (wet) tank

The supply or wet tank receives air directly from the compressor and therefore collects the most water vapor and oil before air passes to other tanks.

Question 8: What is the purpose of a jake brake (engine compression brake)?

To use engine compression to slow the vehicle, reducing reliance on service brakes (Correct answer)
To prevent the engine from over-revving
To increase engine power on upgrades
To automatically apply trailer brakes on downgrades

Correct answer: To use engine compression to slow the vehicle, reducing reliance on service brakes

An engine compression brake uses engine compression to retard vehicle speed, reducing heat buildup in service brakes on long downgrades.

Question 9: A driver is performing a pre-trip inspection and tests the low air pressure warning device. At what pressure should the warning light and buzzer typically activate?

Between 20 and 40 psi
At or above 60 psi (Correct answer)
Below 80 psi
At or above 100 psi

Correct answer: At or above 60 psi

The low air pressure warning light and buzzer must come on at a pressure of 60 psi or higher (or at the manufacturer's specified level, which is typically at or above 60 psi) to alert the driver of low air pressure in the tanks.

Question 10: Compared to traditional S-cam drum brakes, what is a primary advantage of air disc brakes?

They are less prone to brake fade because they dissipate heat more efficiently. (Correct answer)
They have a longer service life due to thicker friction material.
They are less expensive to manufacture and replace.
They require more frequent manual adjustments to maintain performance.

Correct answer: They are less prone to brake fade because they dissipate heat more efficiently.

Air disc brakes have an open design that allows them to dissipate heat more effectively than enclosed drum brakes. This makes them less susceptible to brake fade, which is a reduction in stopping power caused by overheating, especially during long downhill grades or heavy braking.

Question 11: What is the purpose of the brake drum in an S-cam foundation brake system?

To connect the axle shaft to the wheel hub
To provide a friction surface for brake shoes to press against to slow the wheel (Correct answer)
To store compressed air for emergency braking
To house the air brake chamber assembly

Correct answer: To provide a friction surface for brake shoes to press against to slow the wheel

The brake drum rotates with the wheel and provides the friction surface that brake shoes press against to convert kinetic energy into heat and slow the vehicle.

Question 12: During a static air leakage test on a combination vehicle, what is the maximum permissible pressure drop in one minute after the initial drop?

3 psi (Correct answer)
2 psi
4 psi
5 psi

Correct answer: 3 psi

For a combination vehicle, with the engine off and the brakes released, the air pressure should not drop more than 3 psi in one minute after the initial pressure drop. For a single vehicle, the limit is 2 psi.

Question 13: Can spring brakes and service brakes be applied simultaneously?

Yes, to maximize braking force.
No, it can damage the brake system. (Correct answer)
Only if the parking brake valve is manually released.
Only in emergency braking situations.

Correct answer: No, it can damage the brake system.

Applying both spring brakes (parking/emergency brakes) and service brakes simultaneously is generally not recommended and can be detrimental to the brake system. Spring brakes are for parking or emergency situations when service brakes fail, and their combined application can put excessive stress on components. This can lead to damage or uneven braking, which is why they should not be used together.

Question 14: What activates the parking brakes in an air brake system?

The foot brake pedal.
Hydraulic pressure.
Air pressure from the service system.
Spring pressure when air is removed. (Correct answer)

Correct answer: Spring pressure when air is removed.

In an air brake system, parking brakes are typically 'spring brakes' that are held in the released position by air pressure. When the driver activates the parking brake or if the air pressure drops too low (e.g., in a system failure), the air is released, allowing powerful springs to engage the brakes. This design ensures the vehicle will automatically brake if air pressure is lost.

Question 15: What is the primary purpose of ABS on a vehicle?

To reduce stopping distance.
To increase air pressure in the brake system.
To prevent wheel lock-up during hard braking. (Correct answer)
To replace proper braking techniques.

Correct answer: To prevent wheel lock-up during hard braking.

The primary purpose of ABS (Anti-lock Braking System) is to prevent the wheels from locking up during hard braking. This allows the driver to maintain steering control and vehicle stability, especially on slippery surfaces. While it can sometimes reduce stopping distances, its main function is to prevent uncontrolled skidding and improve control.

Question 16: During an air brake pre-trip inspection on a combination vehicle, you perform the applied leakage test. After the initial pressure drop, what is the maximum allowable decrease in air pressure over a one-minute period?

5 psi
2 psi
4 psi (Correct answer)
3 psi

Correct answer: 4 psi

For a combination vehicle, the maximum acceptable air pressure loss during the one-minute applied leakage test (with the service brake fully applied) is 4 psi. For a single vehicle, the limit is 3 psi.

Question 17: What is the primary purpose of the tractor protection valve, which is checked during a pre-trip inspection?

To prevent the trailer brakes from locking up during hard braking.
To manually apply the trailer brakes independently of the tractor brakes.
To prevent total air loss from the tractor if the trailer breaks away or has a major leak. (Correct answer)
To regulate the air pressure sent to the service brakes.

Correct answer: To prevent total air loss from the tractor if the trailer breaks away or has a major leak.

The tractor protection valve is a critical safety device that closes automatically to prevent air from escaping the tractor's brake system in the event of a trailer breakaway or a severe air leak in the trailer's air lines.

Question 18: In a typical dual air brake system on a two-axle truck, how are the two systems usually split?

One system operates the front axle, and the other operates the rear axle. (Correct answer)
Both systems operate all brakes simultaneously, with one as a backup.
One system operates the left side brakes and the other operates the right side brakes.
One system operates the service brakes and the other operates the parking brake.

Correct answer: One system operates the front axle, and the other operates the rear axle.

Typically, the primary system operates the brakes on the rear axle(s), and the secondary system operates the brakes on the front axle. This separation ensures that if one system fails, the other can still provide braking on at least one axle.

Question 19: Which of the following scenarios describes a failed air pressure buildup test, requiring immediate mechanical attention?

The low air warning buzzer activates at 65 psi.
The air pressure takes 55 seconds to build from 85 to 100 psi. (Correct answer)
The air pressure builds from 85 to 100 psi in 40 seconds.
The governor cuts out at 130 psi.

Correct answer: The air pressure takes 55 seconds to build from 85 to 100 psi.

A key standard for the air brake system is that the compressor must build air pressure from 85 to 100 psi in 45 seconds or less. Taking 55 seconds indicates a potential problem with the compressor or a leak in the system, and thus constitutes a failed test.

Question 20: Which component in a drum brake assembly converts the linear motion of the brake chamber pushrod into rotary motion to apply the brakes?

Brake drum
Anchor pin
Slack adjuster (Correct answer)
Return spring

Correct answer: Slack adjuster

The slack adjuster is the mechanical lever that connects the brake chamber's pushrod to the camshaft. Its primary functions are to transform the linear force of the pushrod into the twisting or rotary motion of the S-cam and to provide a way to adjust for lining wear.

Question 21: What is an automatic slack adjuster (ASA)?

A self-adjusting mechanism that maintains proper brake adjustment without manual intervention (Correct answer)
A device that automatically adjusts the governor cut-out pressure
A computer-controlled system that monitors brake temperature
An ABS component that adjusts brake pressure during skids

Correct answer: A self-adjusting mechanism that maintains proper brake adjustment without manual intervention

An automatic slack adjuster continuously self-adjusts to maintain proper brake lining-to-drum clearance without requiring manual adjustment.

Question 22: What is the purpose of an air dryer in the air brake system?

To increase system pressure
To cool compressed air
To remove moisture and contaminants from the air supply (Correct answer)
To store compressed air

Correct answer: To remove moisture and contaminants from the air supply

The air dryer removes moisture and contaminants from compressed air to prevent water buildup and freezing in the system.

Question 23: What is the primary function of the air compressor in an air brake system?

To release air from the brake chambers
To pressurize the brake pedal
To pump air into the storage tanks (Correct answer)
To control the governor cut-out pressure

Correct answer: To pump air into the storage tanks

The air compressor pumps air into the storage tanks to maintain system pressure needed for braking.

Question 24: Which of the following steps is performed to test that the spring brakes will engage automatically?

Apply the service brake firmly while moving at 5 mph.
Build the air pressure to the governor cut-out.
Gently pull against the parking brake in a low gear.
Pump the brake pedal to reduce air pressure until the parking brake valve pops out. (Correct answer)

Correct answer: Pump the brake pedal to reduce air pressure until the parking brake valve pops out.

To check the automatic application of the spring brakes, the driver must fan the brake pedal to lower the air pressure. The parking brake valve (and tractor protection valve on combination vehicles) should pop out between 20 and 45 psi, engaging the spring brakes.

Question 25: What is brake lag distance in an air brake system?

The distance traveled while air pressure builds up to physically apply the brakes after the pedal is pressed (Correct answer)
The distance traveled after brakes are fully released
The extra stopping distance required on wet or slippery roads
The distance traveled while the driver decides to apply the brakes

Correct answer: The distance traveled while air pressure builds up to physically apply the brakes after the pedal is pressed

Brake lag is the distance the vehicle travels while air pressure builds to actually apply the brakes after the pedal is pressed, which can be up to 0.4 seconds.

CDL Air Brakes Knowledge Test

The CDL Air Brakes knowledge test is required for commercial drivers operating vehicles equipped with air brakes. It tests knowledge of air brake system components, dual air brake systems, inspection procedures, and safe operation techniques.

Questions

60 min

Time Limit

80%

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These practice questions were written with AI tools. They are not real exam questions. Exam facts are checked against the official sources listed below. Not reviewed by a subject-matter expert. Editorial policy

Sources & references

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