CDL Practice Tests For Air Brakes Page 5

Air Brakes Practice Questions

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The manual adjustment of automatic slack adjusters is:
  • Dangerous
  • Required
  • Recommended
  • Up to the driver
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From The CDL Manual

The manual adjustment of automatic slack adjusters is dangerous because it gives the vehicle operator a false sense of security about the effectiveness of the braking system.

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When inspecting air brakes:
  • Mechanical parts must be in place, not missing or broken.
  • Linings must not be dangerously thin, loose, or soaked with oil or grease.
  • All of these should be checked for
  • Cracks longer than one-half the width of the friction area are not allowed on drums or discs
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From The CDL Manual

Check brake drums or discs, linings and hoses:

Brake drums or discs must not have cracks longer than one-half the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing. Check the air hoses connected to the brake chambers to make sure they are not cut or worn due to rubbing.

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All of the following apply if the low air pressure warning signal does not work except:
  • You would not know if the vehicle had low air pressure
  • It could cause sudden emergency braking
  • Finish your trip, and have it fixed when you get to it
  • Stopping distance could increase
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From The 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 airpressure 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.

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The low air pressure warning signal indicates:
  • That the air brake system has insufficient air to operate properly
  • A low air warning could signal any or all of these things
  • That air pressure has fallen below 60 psi
  • You may not be able to stop the vehicle properly
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From The CDL Manual

A low air-pressure warning signal is required on vehicles with air brakes. 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). The warning is usually a red light. A buzzer also may come on.

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When should the spring brakes activate?
  • 80 psi
  • Between 20 and 40 psi
  • 120 psi
  • When you turn the truck off
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From The 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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To make sure that the spring brakes come on automatically, do all of the following except:
  • Achieve sufficient air pressure
  • Release the parking brakes
  • Turn the engine on
  • Pump the brake pedal
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From The 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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In dual air systems, how long should it take air pressure to build 85 to 100 psi?
  • 120 seconds
  • 3 minutes
  • 45 seconds
  • 2 minutes
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From The CDL Manual

Check rate of air pressure buildup:

When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer’s specifications.) In single air systems (pre-1975), typical requirements are pressure build-up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 RPM.

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When checking the air pressure buildup, pressure should build:
  • To 85 psi
  • From 85 to 100 psi
  • From 20 to 40 psi
  • To 125 psi
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From The CDL Manual

Check rate of air pressure buildup:

When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer’s specifications.) In single air systems (pre-1975), typical requirements are pressure build-up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 RPM.

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Initial air leakage rate for combination vehicles should be:
  • Less than 2 psi in 1 minute
  • Less than 3 psi in 3 minutes
  • Less than 1 psi in 2 minutes
  • Less than 3 psi in 1 minute
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From The 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.

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When testing air leakage rates for single vehicles, initial loss rate should be less than:
  • 2 psi in 1 minute
  • 1 psi in 2 minutes
  • 3 psi in 1 minute
  • 4 psi in one minute
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From The 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.

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