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5.1 – The Parts of an Air Brake System (continued)

5.1.5 – Alcohol Evaporator

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).

5.1.6 – Safety Valve

A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.

5.1.7 – The Brake Pedal

You apply the brakes by pushing down the brake pedal (also called the “foot valve” or “treadle valve”). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.

Multiple-Choice Questions:

Question #241 (1 of 3)

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Which of the following are true about the pressure relief valve in an air brake system?

  • All these are correct
  • Protects the tank and the rest of the system from too much pressure.
  • Is normally set to open at 150 psi
  • If it releases air, something is wrong. Have the fault fixed by a mechanic.
A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.
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Question #240 (2 of 3)

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Some air brake systems have an alcohol evaporator to put alcohol into the air system. What purpose does this serve?

  • All these answers are correct
  • Under high pressure the air can become stagnant, causing a loss of braking force
  • Helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.
  • It lubricates the rubber seals throughout the air brake system
Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.
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Question #242 (3 of 3)

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What happens when you release the brake (take your foot off the brake) on an air brake system?

  • It lets some compressed air go out of the system, so the air pressure in the tanks is reduced. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it.
  • The air pressure in the system increases, causing the S-cam to turn and apply the brakes
  • None of these are true
  • The air pressure builds beyond the governed limit for a very short time to "reset" the system before reducing the pressure back to the governed pressure
Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it.
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