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CDL Practice Test: Air Brakes

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CDL Practice Test: Air Brakes

Air Brakes Questions

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Good Luck!

Which of the following statements about Dual Air Brake Systems is false?
  • A dual air brake system has two separate air brake systems that use a single set of brake controls
  • The first system is called the "parent" system an the other is called the "child" system
  • Most newer heavy-duty vehicles use dual air brake systems
  • The warning light and buzzer should come on before the air pressure drops below 60 psi in either system
This is a question from page 44 - click here to look up the answer

Quote From Page 65 Of The Illinois CDL Manual:

Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer if there is one. The first system is called the "primary" system. The other is called the "secondary" system.

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges or needles if the system has two needles in one gauge. Pay attention to the low air-pressure warning light and buzzer. The warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi.

The warning light and buzzer should come on before the air pressure drops below 60 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brake system fixed.

TruckingTruth's Advice:

It is very likely the vehicle you drive will have a dual air brake system, so be sure to understand how the system works not only for your exams, but for when you are performing your daily job duties.

Next
Which of the following statements about Brake Fade is incorrect?
  • While brake fade was once a very common problem, it is very rare with modern brake cooling mechanisms
  • Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all
  • Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums
  • As overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

Brakes are designed so brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.


Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

Brake fade also is affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.

TruckingTruth's Advice:

While engine brakes are a fantastic tool in helping to prevent brake fade, the number one cause of brake failure in hilly conditions is brake fade.

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Next
What is a "safe speed"?
  • 10mph under the posted speed limit is considered a safe speed
  • Speeds of 55mph or below are considered safe speeds for maximum fuel mileage
  • A predetermined speed for descending down a long or steep grade, which helps determine when brakes should be applied or released
  • The yellow signs before curves and ramps are designated safe speeds
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

  • 1. Apply the brakes just hard enough to feel a definite slowdown.
  • 2. When your speed has been reduced to approximately 5 mph below your "safe" speed, release the brakes. (This brake application should last about 3 seconds.)
  • 3. When your speed has increased to your "safe" speed, repeat steps 1 and 2.
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Next
The following statements accurately describe Wedge Brakes, except:
  • All of these answers are accurate
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes
  • All wedge-type brakes are self-adjusting
  • Wedge brakes may have either a single brake chamber or two brake chambers
This is a question from page 42 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

Wedge brakes: In these types of brakes, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge-type brakes may be self-adjusting or may require manual adjustment.

TruckingTruth's Advice:

Wedge brakes are derived from old technology and are very rarely used in modern commercial vehicles. However, you still may be asked a question or two about wedge brakes on your written exam.

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Next
All of the following indicate a truck is equipped with an Anti-Lock Brake System, except:
  • Vehicles that have anti-lock brakes have a yellow light near the driver's rear side of the vehicle with the letters ABS stenciled above the light
  • If there is a malfunction in the ABS system, a yellow warning lamp will illuminate and stay on
  • Once the driver turns on the ignition, a yellow malfunction lamp on the instrument panel will light up, briefly indicating that the vehicle has anti-lock brakes
  • All trucks and trailers are required to be equipped with Anti-Lock brake systems
This is a question from page 46 - click here to look up the answer

Quote From Page 67 Of The Illinois CDL Manual:

Vehicles that have anti-lock brakes have a yellow light near the driver's rear side of the vehicle with the letters ABS stenciled above the light. Once the driver turns on the ignition, a yellow malfunction lamp on the instrument panel will light up, briefly indicating that the vehicle has anti-lock brakes. This lamp will remain constant if there is a malfunction in the anti-lock brake system.

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Next
How do you properly use Anti-Lock Brakes?
  • Anti-Lock Brakes are only used when there is a significant amount of brake fading
  • Drivers should use Stab Braking in which they depress the brake, release when the wheels lock up, then depress the brake again repeating as many times as necessary
  • The driver's foot remains on the brake pedal in which the anti-lock module then acts as a foot pumping the air brake system
  • None of these answers are correct
This is a question from page 46 - click here to look up the answer

Quote From Page 67 Of The Illinois CDL Manual:

For normal or emergency stopping using anti-lock brakes, the driver's foot remains on the brake pedal in which the anti-lock module then acts as a foot pumping the air brake system. On the air-brake system the driver must pump or use stab braking in an emergency. If the anti-lock brake system fails or malfunctions, the driver must resort to stopping the vehicle by using the normal air-brake method. If an emergency arises, the driver should use the controlled or stab braking method. The anti-lock brake system should be serviced as soon as possible.

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Next
What is the maximum allowable air loss rate for a single vehicle while the service brakes are released?
  • Less than 2 psi in 1 minute
  • Less than 4 psi in 1 minutes
  • Less than 1 psi in 1 minute
  • Less than 3 psi in 1 minute
This is a question from page 45 - click here to look up the answer

Quote From Page 66 Of The Illinois CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

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Next
What is the purpose of Air Tank Drains?
  • They allow water to be released from the air tanks
  • They allow air to be released when the air pressure builds too high
  • They are used to transfer air from the air tank to the brake chambers
  • They allow oil to be inserted into the air tanks
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

Air Tank Drains - Compressed air usually has some water and compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure to drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types of drain valves:

  • 1. Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
  • 2. Automatic, in which the water and oil are automatically expelled. They may be equipped for manual draining as well.

The automatic types are available with electric heating devices. These help prevent freeze-up of the automatic drain in cold weather.

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