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Section 5: Air Brakes

This Section Covers:

  • Air Brake System Parts
  • Dual Air Brake Systems
  • Inspecting Air Brakes
  • Using Air Brakes

This section tells you about air brakes. If you want to drive a truck or bus with air brakes, or pull a trailer with air brakes, you need to read this section. If you want to pull a trailer with air brakes, you also need to read Section 6, Combination Vehicles.

Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles, but the brakes must be well maintained and used properly.

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.

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

The parts of these systems are discussed in greater detail below.

5.1 – The Parts of an Air Brake System

There are many parts to an air brake system. You should know about the parts discussed here.

5.1.1 – 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.

5.1.2 – 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.

5.1.3 – Air Storage Tanks

Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.

5.1.4 – Air Tank Drains

Compressed air usually has some water and some 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 that you drain the air tanks completely.

Each air tank is equipped with a drain valve in the bottom. There are two types:

  • 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.
  • Automatic: the water and oil are automatically expelled. These tanks may be equipped for manual draining as well.

Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather.

Multiple-Choice Questions:

Question #232 (1 of 8)

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Air brakes are really three different systems:

  • S-cam brake, engine brake, and emergency brake
  • Emergency brake, power brake, and parking brake
  • Service brake, power brake, and emergency brake
  • Service brake, parking brake, and emergency brake

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.

  • The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
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Question #234 (2 of 8)

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The parking brake system:

  • Controls when the air compressor will pump air into the air storage tanks
  • Uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
  • Applies and releases the brakes when you use the brake pedal during normal driving.
  • Applies and releases the parking brakes when you use the parking brake control.

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.

  • The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
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Question #238 (3 of 8)

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Which component is used to hold compressed air and will hold enough air to allow the brakes to be used several times, even if the compressor stops working?

  • Air Compressor gate
  • Service pump
  • Air storage tanks
  • Governor

Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.

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Question #235 (4 of 8)

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The emergency brake system:

  • Applies and releases the brakes when you use the brake pedal during normal driving.
  • Uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
  • Applies and releases the parking brakes when you use the parking brake control.
  • Controls when the air compressor will pump air into the air storage tanks

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.

  • The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
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Question #233 (5 of 8)

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The service brake system:

  • Applies and releases the parking brakes when you use the parking brake control.
  • Uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
  • Applies and releases the brakes when you use the brake pedal during normal driving.
  • All these are correct

Air brakes are really three different braking systems: service brake, parking brake, and emergency brake.

  • The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
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Question #237 (6 of 8)

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Which component controls when the air compressor will pump air into the air storage tanks?

  • Governor
  • Air Compressor gate
  • Accentuator
  • Service pump

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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Question #239 (7 of 8)

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Why do air tanks have a drain valve?

  • Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system.
  • It's impossible to create an automatic drain valve so a manual one must be installed
  • As a backup in case the governor quits, the drain acts as a pressure relief valve
  • All these answers are correct

Compressed air usually has some water and some 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 that you drain the air tanks completely.

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Question #236 (8 of 8)

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Which component pumps air into the air storage tanks?

  • Air compressor
  • Governor
  • Accentuator
  • Service pump

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.

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