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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!

When will spring brakes automatically activate?
  • Spring brakes should never activate automatically
  • When air pressure drops to a range of 20 to 45 psi
  • When air pressure drops to a range of 100 to 120 psi
  • When air pressure is completely depleted
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

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:

Remember, spring brakes are used as both parking brakes as well as emergency brakes. If you begin losing air pressure, it is important to pull over quickly to a safe area as your spring brakes will automatically activate once air pressure drops too low.

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In a Dual Air Brake System:
  • The low air warning should not activate until 30 psi since there are two air brake systems
  • The low air warning should only activate when both systems drop below 60 psi
  • The low air warning should activate before pressure drops below 60 psi in either system
  • The low air warning should activate before 120 psi since it's a dual air brake system
This is a question from page 44 - click here to look up the answer

Quote From Page 65 Of The Illinois CDL Manual:

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:

Regardless of whether your vehicle has a single air brake system or a duel air brake system, if any air pressure gauge reads below 60 psi, you should have a warning about low air pressure even if the second system is fully charged.

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Next
What is a Wig-Wag?
  • A new technology using GPS which automatically fills the air tanks when a steep decline is ahead
  • This is a type of supply pressure gauge which normally hangs from near the visor
  • It is located inside of the brake chamber and records any mechanical failures
  • In older vehicles, this is a mechanical type of low air pressure warning indicator
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

Another type of warning is the "wig wag." This device drops a mechanical arm into your view when the pressure in the system drops below 60 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 60 psi. The manual reset type must be placed in the "out of view" position manually. It will not stay in place until the pressure in the system is above 60 psi.

TruckingTruth's Advice:

Except in very old commercial vehicles, the Wig-Wag warning device is about extinct and is no longer used. Simply know what it is as there may be a question asked about it on the written exam. It is likely you will never encounter a wig wag warning device in real life.

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What is the service brake?
  • A system used by mechanics during servicing do deactivate the emergency brakes
  • A system which applies and releases the parking brakes when you use the parking brake control
  • A system which uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.
  • A system which applies and releases the brakes when you use the brake pedal during normal driving
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 vital that you learn the difference between the following definitions:

  • Service Brake
  • Parking Brake
  • Emergency Brake

Memorize each definition and be sure you understand how each of these differ. Not only will this be critically important to passing the air brakes endorsement test but it is terminology you'll need to understand in the real world.

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Next
For an average driver traveling 55mph under good traction and brake conditions, total stopping distance using air brakes will have a stopping distance of:
  • More than 400 feet
  • More than 550 feet
  • More than 300 feet
  • More than 250 feet
This is a question from page 46 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, for an average driver traveling 55 mph under good traction and brake conditions, the total stopping distance is more than 300 feet. This is longer than a football field.

TruckingTruth's Advice:

Brake lag distance isn't only something you'll see in a CDL manual or training course. This is a real issue and one of the biggest downfalls to an air brake system.

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Next
Depressing the brake pedal activates which brake system?
  • The emergency brakes
  • The service brakes
  • The parking brakes
  • The driving 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
At what psi will the air compressor governor tell the air compressor to "cut-in"?
  • 90 psi
  • 120 psi
  • 110 psi
  • 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:

Check air compressor governor cut-in and cut-out pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.

TruckingTruth's Advice:

It is important to memorize the "cut in" and "cut out" levels for the air governor as well as the terminology.

Cut in: When the governor tells the air compressor to start pumping air into the air tanks. This should occur when air pressure falls to about 100 psi.

Cut out: When the governor tells the air compressor to stop pumping air into the air tanks. This should occur when the air pressure has risen to about 125 psi.

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Next
Why do air tanks need to be manually drained?
  • The brake system sends water through the brake lines to flush the air lines out and the water needs to be drained through the air tanks before another system flush can occur
  • Compressed air causes water to form and condense which is bad for the air brake system, especially if it freezes during cold weather
  • Draining the air tanks helps to ensure the air pressure isn't building too high in the tanks
  • Draining air tanks helps bring the air pressure lower which can then be used to test the air governor
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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