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CDL Practice Test: Air Brakes

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CDL Practice Test: Air Brakes

Air Brakes Questions

Click On The Picture To Begin

Good Luck!

What is "Cut-In Air Pressure"?
  • When the governor tells the air compressor to stop pumping air into the air tanks
  • When the governor tells the service brakes to activate
  • When the governor tells the air compressor to start pumping air into the air tanks
  • When the governor tells the emergency brakes to engage
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.

Next
At what psi must the low air pressure warning indicator activate?
  • Before dropping below 60 psi
  • Before dropping below 20 psi
  • Before dropping below 40 psi
  • Before dropping below 80 psi
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles).

TruckingTruth's Advice:

On large busses, it is common for the low-pressure warning devices to signal at 80 to 85 psi, but that is an additional safety feature. Regulations require the warning to activate before 60 psi. This is very important to memorize not only for testing, but for everyday job duties.

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Next
The air compressor governor will begin supplying air around what PSI?
  • Around 100 psi
  • Around 75 psi
  • Around 50 psi
  • Around 125 psi
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

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:

Make sure you memorize the air pressure for when the governor will "cut in" and "cut out" as you will need to know this for the written exam, the pre-trip exam, and in the real world while checking your gauges going down the road.

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Next
What is a "safe speed"?
  • A predetermined speed for descending down a long or steep grade, which helps determine when brakes should be applied or released
  • 10mph under the posted speed limit is considered a safe speed
  • The yellow signs before curves and ramps are designated safe speeds
  • Speeds of 55mph or below are considered safe speeds for maximum fuel mileage
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
Which of the following statements is true about Air Tank Drains?
  • Manually operated air tank drains normally use a handle or a cable to drain the system
  • Automatic air tank drains always have to be drained manually as well
  • All fluids from all air tanks drain into a central location which must be drained
  • Air tank drains should be forced into the open position during dry weather so that they drain throughout the day
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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Next
When should air tanks be drained?
  • Every month
  • At least once per week
  • None of these answers are correct
  • At the end of each day
This is a question from page 47 - click here to look up the answer

Quote From Page 69 Of The Illinois CDL Manual:

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.

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Next
What are foundation brakes?
  • Foundation brakes are used at each wheel and make up the individual components of the brake system
  • Foundation brakes are also known as the emergency brakes
  • Foundation brakes are used for long-term parking when it is believed the air pressure will deplete over time
  • Foundation brakes are the brakes used when depressing the brake pedal
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
What happens when the brake drums exceed their designed temperature?
  • Too much heat can make the brakes stop working
  • Too much heat will cause the brakes to "catch" and potentially lock up the brakes
  • Too much heat will force the emergency brakes to engage
  • Too much heat will activate the water-cooled emergency system
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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