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CDL Practice Test: Air Brakes

These CDL practice questions are from our High Road Training Program, a CDL test preparation course designed to help you learn the CDL manual.

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CDL Practice Test: Air Brakes

Air Brakes Questions

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Good Luck!

What is the maximum allowable air loss rate for a combination vehicle with the service brakes released?
  • Less than 1 psi in 1 minute
  • Less than 2 psi in 1 minute
  • There should be no loss of any 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!!

Next
What is a Front Brake Limiting Valve?
  • All of these accurately describe a Front Brake Limiting Valve
  • When you put the control in the "slippery" position, the limiting valve cuts the "normal" air pressure to the front brakes by half
  • These are only installed on some vehicle made before 1975
  • Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

Some older vehicles (made before 1975) have a front brake limiting valve and a control in the cab. The control is usually marked "normal" and "slippery." When you put the control in the "slippery" position, the limiting valve cuts the "normal" air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front-wheel braking is good under all conditions. Tests show that front-wheel skids from braking are not likely even on ice. Make sure the control is in the "normal" position to have normal stopping power.

Many vehicles have automatic front-wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure). These valves cannot be controlled by the driver.

TruckingTruth's Advice:

It is extremely unlikely you will ever drive a vehicle with front brake limiting valves. However, there is a chance you may be asked a question about this on the written exam.

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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
What happens when the brake drums exceed their designed temperature?
  • Too much heat will cause the brakes to "catch" and potentially lock up the brakes
  • Too much heat will activate the water-cooled emergency system
  • Too much heat will force the emergency brakes to engage
  • Too much heat can make the brakes stop working
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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Next
What does it mean for the air compressor to "cut in"?
  • The air compressor will stop releasing air from the air storage tanks
  • The air compressor will stop supplying air to the air storage tanks
  • The air compressor will begin supplying air to the air storage tanks
  • The air compressor will begin releasing air from the air storage 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 Compressor - The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

Air Compressor Governor - 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.

TruckingTruth's Advice:

Be sure to understand how the air compressor and air compressor governor both work. The air compressor governor does not supply air, rather, it tells the air compressor when it should or shouldn't be supplying air to the air storage tanks.

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Next
All of the following indicate a truck is equipped with an Anti-Lock Brake System, except:
  • 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
  • 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
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
What does an Alcohol Evaporator do?
  • Keeps the air lines free from sand, dirt, and dust
  • Supplies an alcohol mixture to the brake pads, keeping them clean from foreign debris
  • Helps reduce the risk of ice in air brake valves and other parts during cold weather
  • Injects alcohol into the air compressor, keeping moving parts clean and free from debris
This is a question from page 41 - click here to look up the answer

Quote From Page 63 Of The Illinois CDL Manual:

Alcohol Evaporator - Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).

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Next
Before driving a vehicle with a dual air brake system, the air pressure should be above what psi?
  • 60 psi
  • 100 psi
  • 120 psi
  • 80 psi
This is a question from page 44 - click here to look up the answer

Quote From Page 65 Of The Illinois CDL Manual:

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

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