CDL Air Brakes Endorsement Test — Questions and Answers

Question 1: What is the purpose of the safety relief valve in an air brake system?

To release excess air if the governor fails and pressure becomes dangerously high (Correct answer)
To regulate air flow to individual wheel cylinders
To drain moisture from the air tanks
To maintain constant pressure in the brake lines

Correct answer: To release excess air if the governor fails and pressure becomes dangerously high

The safety relief valve is a backup protection device that opens to vent air if the governor malfunctions and pressure exceeds a safe limit (usually set at 150 psi).

Question 2: What should a driver do if an air leak is found during the pre-trip inspection?

Use the brakes only when absolutely necessary.
Report the air leak and avoid using the brake system. (Correct answer)
Ignore the air leak and proceed with the trip.
Continue driving but avoid using the brakes.

Correct answer: Report the air leak and avoid using the brake system.

An air leak in the brake system is a serious safety hazard because it can lead to a rapid loss of air pressure, compromising the vehicle's ability to brake effectively. Continuing to drive with an air leak is extremely dangerous and could result in complete brake failure. Therefore, reporting the leak and ensuring it is repaired before operation is a critical safety measure.

Question 3: During a brake adjustment check, you measure the pushrod stroke with the brakes applied at approximately what air pressure?

90 psi (Correct answer)
60 psi
120 psi
100 psi

Correct answer: 90 psi

Pushrod stroke is measured with full service brake application at approximately 90 psi to get a consistent and accurate reading of the slack condition.

Question 4: What is one of the key emergency procedures to follow when a brake failure occurs?

Activate the emergency brake and stop the vehicle in a safe area. (Correct answer)
Speed up to reach a safe area faster.
Call the supervisor before stopping the vehicle.
Continue driving while applying the foot brake.

Correct answer: Activate the emergency brake and stop the vehicle in a safe area.

When a brake failure occurs, one of the key emergency procedures is to activate the emergency brake and bring the vehicle to a stop in a safe area. The emergency brake often operates independently of the main air brake system, providing a crucial backup. This action helps to regain some control and safely stop the vehicle, preventing a potentially catastrophic runaway situation.

Question 5: What is the role of the trailer breakaway valve in an emergency separation?

It shuts off airflow to the tractor brakes to preserve pressure
It releases all trailer brakes when the trailer detaches
It prevents the glad hand connections from separating during normal driving
It automatically applies the trailer brakes if the trailer becomes disconnected from the tractor (Correct answer)

Correct answer: It automatically applies the trailer brakes if the trailer becomes disconnected from the tractor

The breakaway valve uses a pull cord connected to the tractor; if the trailer separates, the cord pulls the valve, cutting off air to the trailer service line and applying the trailer spring brakes.

Question 6: How should you test the service brakes during a pre-trip air brake check?

Pump the pedal 20 times while parked
Roll forward slowly and apply the brake pedal firmly (Correct answer)
Disconnect the trailer line
Rev the engine to build pressure

Correct answer: Roll forward slowly and apply the brake pedal firmly

Move forward at low speed and apply the brakes to confirm they stop the vehicle and do not pull.

Question 7: Which condition is MOST likely to cause brake fade on a long downhill grade?

Checking air pressure before beginning the descent
Engaging the engine brake at the top of the grade
Using the service brakes in short controlled applications
Continuous heavy braking that overheats the brake drums and linings (Correct answer)

Correct answer: Continuous heavy braking that overheats the brake drums and linings

Continuous heavy braking generates heat that raises drum and lining temperatures until the friction material can no longer effectively slow the vehicle, causing brake fade.

Question 8: What happens if the air pressure in the system drops too low?

Cab lights will turn off
Engine will overheat
Brakes will automatically apply (Correct answer)
Horn will stop working

Correct answer: Brakes will automatically apply

Air brake systems are designed with a critical safety feature: if the air pressure drops too low (typically below 20-45 psi), the spring brakes will automatically engage. This mechanism ensures that the vehicle can still be brought to a stop, preventing runaway situations even in the event of a major air leak or system failure, thereby enhancing safety.

Question 9: What is the air brake endorsement requirement for CDL holders who drive vehicles with air brakes?

Only Class A CDL holders need air brake authorization
No specific authorization is needed beyond the base CDL
Drivers need a separate Air Brake Endorsement added to their CDL
Drivers must not have an air brake restriction (L restriction) on their CDL, obtained by passing the air brake knowledge and skills tests (Correct answer)

Correct answer: Drivers must not have an air brake restriction (L restriction) on their CDL, obtained by passing the air brake knowledge and skills tests

CDL applicants who fail the air brake knowledge test or are tested in a vehicle without air brakes receive an L restriction, prohibiting them from driving vehicles with air brakes.

Question 10: How can one minimize brake lag and apply the brakes more quickly by keeping an air supply close to the rear service brakes?

Rear reservoir
Supply valve
Relay valve (Correct answer)
Automatic slack adjuster

Correct answer: Relay valve

A relay valve is used in air brake systems, particularly on longer vehicles or trailers, to reduce brake lag. It acts as a remote-controlled valve, allowing a large volume of air from a nearby reservoir to be quickly directed to the brake chambers. This minimizes the time it takes for air to travel from the foot valve to the rear brakes, ensuring a more immediate and effective brake application.

Question 11: What is total stopping distance for an air brake vehicle made up of?

Air pressure and tire size
Only the braking distance
Perception, reaction, brake lag, and effective braking distance (Correct answer)
Reaction distance only

Correct answer: Perception, reaction, brake lag, and effective braking distance

Air brake stopping distance adds brake lag distance to perception, reaction, and effective braking distance.

Question 12: How can a driver test the adjustment of the air brakes?

By measuring the temperature of the brakes.
By listening for unusual noises from the engine.
By checking the brake fluid levels.
By performing a manual brake test and ensuring proper engagement. (Correct answer)

Correct answer: By performing a manual brake test and ensuring proper engagement.

To test the adjustment of air brakes, a driver should perform a manual brake test, often involving checking the slack adjusters. This process allows the driver to physically verify that the brake shoes are moving the correct distance and engaging properly with the brake drums. Ensuring proper engagement confirms that the brakes will apply effectively and evenly when needed, which is critical for safe stopping.

Question 13: What legal requirement must a driver meet regarding air brake inspections?

Perform inspections only after accidents.
Inspect the system once a year.
Conduct pre-trip and post-trip inspections to ensure safety and compliance. (Correct answer)
Only perform inspections when the vehicle shows signs of malfunction.

Correct answer: Conduct pre-trip and post-trip inspections to ensure safety and compliance.

Legal requirements mandate that drivers perform thorough pre-trip and post-trip inspections of their air brake systems. These routine checks are crucial for identifying any potential malfunctions or issues before they lead to dangerous situations on the road. This proactive approach ensures the vehicle's safety, compliance with transportation regulations, and reduces the risk of accidents.

Question 14: You notice the air pressure gauge dropping slowly while the vehicle is parked with the engine off. What does this indicate and what should you do?

This is normal — small pressure losses while parked are expected and acceptable
Start the engine immediately to rebuild pressure before the warning buzzer activates
This indicates an air leak that must be found and repaired before the vehicle is operated (Correct answer)
Drain and refill all air tanks to reset the system before departing

Correct answer: This indicates an air leak that must be found and repaired before the vehicle is operated

Pressure should hold steady when parked; a slow drop confirms air is escaping through a leak that must be repaired before safe operation.

Question 15: What does it mean if the low air pressure warning activates while checking brake adjustment during a pre-trip?

Slack adjusters are over-tightened
Brake lining thickness is at the minimum
The air compressor or governor may not be functioning properly (Correct answer)
The spring brakes need adjustment

Correct answer: The air compressor or governor may not be functioning properly

If the low air pressure warning activates during testing, it signals that the compressor is not maintaining adequate system pressure, pointing to a problem with the compressor or governor.

Question 16: When the low air pressure warning light comes on, at roughly what pressure does it typically activate?

Below 120 psi
Below 150 psi
Below 60 psi (Correct answer)
Below 100 psi

Correct answer: Below 60 psi

The low air pressure warning must activate before pressure drops below 60 psi.

Question 17: At 55 mph on dry pavement, approximately how many feet of additional stopping distance does air brake lag add compared to the moment you press the brake pedal?

About 100 feet
About 10 feet
About 60 feet
About 32 feet (Correct answer)

Correct answer: About 32 feet

At 55 mph, a vehicle travels roughly 80 feet per second, so 0.4 seconds of brake lag adds approximately 32 feet before braking even begins.

Question 18: Which condition would cause the air dryer purge valve to cycle more frequently than normal?

The governor being set too high
Excessive moisture entering the system due to a failing compressor or hot weather (Correct answer)
Low ambient temperature freezing the desiccant cartridge
Air leaks in the brake chambers

Correct answer: Excessive moisture entering the system due to a failing compressor or hot weather

High moisture levels from a worn compressor or humid conditions cause the air dryer to purge more frequently as it works to remove the excess water from the system.

Question 19: What is the first step in an emergency braking situation?

Turn off the engine to stop the vehicle.
Release the brake pedal to reset the system.
Apply firm and steady pressure to the brake pedal. (Correct answer)
Apply the handbrake immediately.

Correct answer: Apply firm and steady pressure to the brake pedal.

In an emergency braking situation, the first and most crucial step is to apply firm and steady pressure to the brake pedal. This action maximizes the braking force and allows the anti-lock braking system (ABS), if present, to function optimally by preventing wheel lock-up. Pumping the brakes or applying them lightly can reduce stopping effectiveness and increase stopping distance.

Question 20: How can drivers ensure they are compliant with air brake system regulations during inspections?

By reporting air brake issues to regulatory authorities.
By performing regular inspections and following safety procedures. (Correct answer)
By focusing on engine performance only.
By ensuring all air brake system issues are addressed after inspection.

Correct answer: By performing regular inspections and following safety procedures.

Drivers ensure compliance during air brake system inspections by consistently performing thorough checks and adhering to established safety procedures. This proactive approach allows for the early detection of any potential issues or malfunctions, ensuring that the system meets all regulatory requirements before the vehicle is operated. Regular, procedural inspections are key to maintaining safety and avoiding violations.

Question 21: What is 'brake fade' and when does it most commonly occur?

Automatic reduction of brake force by the ABS system
Loss of air pressure due to a system leak
Gradual wear of brake linings over tens of thousands of miles
Reduced braking effectiveness caused by overheated friction materials (Correct answer)

Correct answer: Reduced braking effectiveness caused by overheated friction materials

Brake fade occurs when friction materials overheat, reducing their ability to create the friction needed to slow the vehicle.

Question 22: What is the primary function of the air pressure regulator in an air brake system?

To regulate and maintain air pressure within safe limits. (Correct answer)
To increase the speed of the braking process.
To release air when needed.
To filter contaminants from the air.

Correct answer: To regulate and maintain air pressure within safe limits.

The primary function of the air pressure regulator in an air brake system is to control and maintain the air pressure within safe and optimal operating limits. It ensures that the compressor does not over-pressurize the system and that a consistent pressure is supplied to the brake components. This regulation is crucial for preventing damage and ensuring effective braking.

Question 23: What is the typical 'cut-out' pressure for an air brake compressor governor?

90 psi
60 psi
150 psi
125 psi (Correct answer)

Correct answer: 125 psi

The governor cuts out (stops the compressor from pumping) at approximately 125 psi, which is the normal maximum operating pressure for air brake systems.

Question 24: Which three fundamental brake types are there?

S cam brakes, disc brakes and wedge brakes. (Correct answer)
S cam brakes, disc brakes and friction brakes.
Drum brakes, disc brakes and wedge brakes.
Rotary brakes, disc brakes and wedge brakes.

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

The three fundamental types of air brakes commonly found on commercial vehicles are S-cam drum brakes, disc brakes, and wedge brakes. S-cam brakes are the most common, using an S-shaped cam to push brake shoes against a drum. Disc brakes use calipers and pads to squeeze a rotor, while wedge brakes use a wedge to force shoes apart against a drum.

Question 25: What is the purpose of inspecting the air tanks during a pre-trip inspection?

To check if the air tanks need to be replaced.
To check for air leaks and ensure proper air storage. (Correct answer)
To ensure the tanks are full of air.
To ensure the tanks are the correct color.

Correct answer: To check for air leaks and ensure proper air storage.

The air tanks are vital components of an air brake system, responsible for storing compressed air needed for braking. Inspecting them during a pre-trip check ensures there are no leaks that could deplete the air supply, and that they are capable of holding sufficient pressure. Proper air storage is fundamental for reliable brake operation and overall vehicle safety.

CDL Air Brakes Endorsement Test

The CDL Air Brakes test certifies a driver's knowledge of air brake system components, operation, inspection, and safe braking procedures for commercial vehicles.

Questions

30 min

Time Limit

80%

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