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5.4.7 – Low Air Pressure

If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Controlled braking is possible only while enough air remains in the air tanks.

The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi. A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on.

It is much safer to stop while there is enough air in the tanks to use the foot brakes.

5.4.8 – Parking Brakes

Any time you park, use the parking brakes, except as noted below.

Pull the parking brake control knob out to apply the parking brakes and push it in to release. The control will be a yellow, diamond-shaped knob labeled “parking brakes” on newer vehicles. On older vehicles, it may be a round blue knob or some other shape (including a lever that swings from side to side or up and down).

Do not use the parking brakes if the brakes are very hot (from just having come down a steep grade) or if the brakes are very wet in freezing temperatures. If they are used while they are very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move. Use wheel chocks on a level surface to hold the vehicle. Let hot brakes cool before using the parking brakes. If the brakes are wet, use the brakes lightly while driving in a low gear to heat and dry them.

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.

Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage.

Multiple-Choice Questions:

Question #304 (1 of 6)

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If the parking brakes are wet on a very cold day:

  • Use them normally
  • Use the brakes lightly while driving in a low gear to heat and dry them.
  • Adjust the brakes properly before setting the parking brake to avoid cracking the brake chamber
  • Do not use the parking brakes at all. Use chocks instead.
If the brakes are wet, use the brakes lightly while driving in a low gear to heat and dry them.
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Question #302 (2 of 6)

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Do not use the parking brakes if the brakes are:

  • Properly adjusted or if the S-cam is turning in the brake chamber
  • None of these are correct
  • Very hot (from just having come down a steep grade) or very wet in freezing temperatures.
  • Very wet on a hot day or if the air pressure is above 100 psi
Do not use the parking brakes if the brakes are very hot (from just having come down a steep grade) or if the brakes are very wet in freezing temperatures.
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Question #301 (3 of 6)

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Which of the following statements are true about the parking brake control knob?

  • Pull the parking brake control knob out to apply the parking brakes and push it in to release. The control will be a red, octagon-shaped knob labeled “trailer air supply” on newer vehicles.
  • None of these are correct
  • Pull the parking brake control knob out to apply the parking brakes and push it in to release. The control will be a yellow, diamond-shaped knob labeled “parking brakes” on newer vehicles.
  • Push the parking brake control knob in to apply the parking brakes and pull it out to release. The control will be a yellow, diamond-shaped knob labeled “parking brakes” on newer vehicles.
Pull the parking brake control knob out to apply the parking brakes and push it in to release. The control will be a yellow, diamond-shaped knob labeled “parking brakes” on newer vehicles.
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Question #300 (4 of 6)

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Which of the following statements are true about spring brakes?

  • Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on.
  • The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi
  • All these are correct
  • A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles.
The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi. A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on.
Remember, the spring brakes are actually your emergency brakes. The two terms can be used interchangeably.
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Question #303 (5 of 6)

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Which of the following are true about parking brakes?

  • If they are used while they are very cold and dry, they can cause the S-cam to seize. If they are used in hot temperatures when the brakes are very wet, the air pressure can build to unsafe levels
  • If they are used in a very dry environment, you must drain the air tanks within one hour of setting the brakes.
  • If they are used while they are very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move.
  • All these are correct
Do not use the parking brakes if the brakes are very hot (from just having come down a steep grade) or if the brakes are very wet in freezing temperatures. If they are used while they are very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move.
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Question #299 (6 of 6)

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The spring brakes will come on when:

  • The brake chamber indicator shows too much air pressure in the canister
  • The air pressure drops into the range of 20 to 45 psi
  • All of these are correct
  • The red air line (emergency brake line) is connected properly
The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi
Remember, the spring brakes are actually your emergency brakes. The two terms can be used interchangeably.
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