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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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What is a dual air brake system
  • A single air brake system that uses two sets of brake controls
  • A system which does not have spring brakes and instead, uses a second S-Cam system for its emergency brakes
  • A fail safe system where when one air brake system fails below 60 psi, the second system will keep the emergency brakes from activating
  • Two separate air brake systems that use a single set of brake controls
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.

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What are Spring Brakes?
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes
  • An S-Cam forces the brake shoes away from one another and presses them against the inside of the brake drum
  • Air pressure acts on a brake chamber and slack adjuster
  • Powerful springs are held back by air pressure and if the air pressure is removed, the springs put on the brakes
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

Spring Brakes - All trucks, truck tractors and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force because air pressure can eventually leak away. Spring brakes are usually used to meet these needs. When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.


Tractor and straight truck spring brakes will come on fully when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi). Do not wait for the brakes to come on automatically. When the low air-pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away while you can still control the brakes.

The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.

TruckingTruth's Advice:

Spring brakes control your parking and emergency brakes. It's a fail-safe system where brakes are held off with air pressure. If you lose air pressure, the brakes will no longer be held back and will automatically activate. That is why it's important to pay attention to your air pressure gauges. If your air pressure drops too low, you need to pull over before the emergency brakes (spring brakes) activate.

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Next
At what psi should the low air warning light and buzzer activate?
  • 120 psi
  • 80 psi
  • None of these are correct
  • 100 psi
This is a question from page 45 - click here to look up the answer

Quote From Page 66 Of The Illinois CDL Manual:

Test low pressure warning signal: Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on. Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure. The  low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems.


If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

TruckingTruth's Advice:

It is very important to know how to check the low pressure warning signal. You will learn most of the procedure when studying for the pre-trip exam, but for the written exam, here's what you should know:



       
  • Build air pressure so the warning light and buzzer are not activated.

  •    
  • Turn the engine off.

  •    
  • Turn the key to the "on" position (this will allow you to see the warning light and see the buzzer).

  •    
  • Begin depressing and releasing the service brake (brake pedal) repeatedly. This will deplete your air pressure and at about 60 psi, the warning light and buzzer should activate.

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What happens when the brake drums exceed their designed temperature?
  • Too much heat can make the brakes stop working
  • Too much heat will force the emergency brakes to engage
  • Too much heat will activate the water-cooled emergency system
  • Too much heat will cause the brakes to "catch" and potentially lock up the brakes
This is a question from page 42 - click here to look up the answer

Quote From Page 63 Of The Illinois CDL Manual:

Too much heat can make the brakes stop working.

TruckingTruth's Advice:

As the brake drums heat up, they become less and less effective. This is a process known as "brake fade" which is a very popular term in the trucking industry. After a while, they will become totally useless. Brake Fade is the #1 cause of brake failure on steep downgrades.

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What does the Air Compressor Governor do?
  • Ensures that air never stops flowing into the air storage tanks
  • Activates the emergency brakes when air pressure gets too high
  • Controls when the air compressor will pump air into the air storage tanks
  • Detects any leaks in the airlines
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level (around 125 pounds per square inch, or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.

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Next
What are foundation brakes?
  • Foundation brakes are used for long-term parking when it is believed the air pressure will deplete over time
  • Foundation brakes are also known as the emergency brakes
  • Foundation brakes are the brakes used when depressing the brake pedal
  • Foundation brakes are used at each wheel and make up the individual components of the brake system
This is a question from page 42 - click here to look up the answer

Quote From Page 63 Of The Illinois CDL Manual:

Foundation brakes are used at each wheel. The most common type is the S-cam drum brake.

TruckingTruth's Advice:

While you may not be asked about the specific term "foundation brakes" you will absolutely need to know the different parts of an S-Cam brake drum. Be sure to study those parts and know what they do. This will be important not only for your written exam, but also your pre-trip exam.

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Next
Depressing the brake pedal activates which brake system?
  • The driving brakes
  • The parking brakes
  • The emergency brakes
  • The service brakes
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

Air brake systems combine three braking systems: service, parking and emergency brake systems.

  • 1. The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • 2. The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • 3. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.

TruckingTruth's Advice:

It is very important that you memorize the difference between the service brake, parking brake, and emergency brake. Study what the function is of each as you'll likely be asked about all 3 on the written exam. These are also terms you will need to know when out in the "real world."

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Next
How can you check that the spring brakes come on automatically?
  • Chock the wheels, turn the engine off, release all brakes, then step on and off the brake pedal to deplete air in the system until the emergency brakes activate
  • With your foot on the service brake, engage and release the parking brake until air is depleted enough for the emergency brake valve to pop out
  • With the engine running and the parking brake on, step on and off the brake pedal to deplete air in the system until the emergency brakes activate
  • Chock the wheels, turn the engine off, release all brakes, then disconnect a brake line and allow the air to deplete which will show if the spring brakes activate automatically
This is a question from page 45 - click here to look up the answer

Quote From Page 66 Of The Illinois CDL Manual:

Check that spring brakes come on automatically: Chock the wheels, release the parking brakes when you have enough air pressure to do it and shut the engine off. Step on and off the brake pedal to reduce the air tank pressure. The "parking brake" knob should pop out when the air pressure falls to the manufacturer's specification, usually in a range between 20 and 40 psi. This causes the spring brakes to come on.

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