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Air Brakes Questions & Answers


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If you plan to operate a commercial vehicle equipped with Air Brakes, you’ll need to pass the air brakes knowledge test and take your skills test in a vehicle that has air brakes. 

Here are some common questions you may find regarding CDL air brakes. 

What does the application pressure gauge do?

The application pressure gauge shows you how much air pressure you are applying to your brakes. 

If you notice an increase in application pressure to hold the same speed, then your brakes are fading. You should slow down and shift to a lower gear. 

A need for increased pressure may point to:

  • Brakes that are out-of-adjustment. 
  • Air leaks.
  • Mechanical issues.

Note that this gauge may not be found in all vehicles. 

How often should you drain the tanks if using air tanks with manually operated drains?

If you have a commercial vehicle with manually operated air tank drains, you should drain the tanks at the end of each day of driving. 

To drain the tanks manually, you’ll typically either turn the valve a quarter turn or pull a cable. 

What does the air compressor governor control?

The air compressor governor controls when the air compressor pumps air into the storage tanks. 

At a high PSI threshold, the governor will hit the “cut-out” level which stops the compressor from pumping air. At a low PSI threshold, the governor will hit the “cut-in” pressure and the compressor will start pumping again. 

What are the maximum leakage rates?

A fully charged air system is typically around 125 PSI. 

The maximum leakage rate you should see should be less than 2 PSI per minute for single vehicles and less than 3 PSI per minute for combination vehicles.

If your leakage rate is higher than those rates, check for air leaks and make any necessary fixes before driving the vehicle. 

What does an alcohol evaporator help prevent?

Certain air brake systems include an alcohol evaporator that adds alcohol to the air system.

The alcohol evaporator prevents ice build up in the brake system in cold weather. Ice build-up inside the brakes can prevent them from working properly. 

What is a dual air brake system?

A dual-air brake system is found on many heavy-duty commercial vehicles as a safety measure. 

A dual air brake system consists of:

  • 2 separate air brake systems.
  • A single set of controls. 
  • One system that operates the brakes on the rear axle (primary system).
  • One system that operates the brakes on the front axle (secondary system).

You can find more information about this topic in the Air Brakes (5) section of your official driver’s handbook. 

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