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How Air Brakes Work And How To Test Them

Understanding how your air brake systems work and how to inspect them is important for every professional driver to know. What is an air brake? An air brake is the mechanism used within a heavy-duty vehicle in order to make it stop. Unlike hydraulic brakes that are used for non-commercial vehicles, air brakes use compressed air to push a piston to apply pressure to the brake pad in order to stop the vehicle.

Compressed air is a necessity for your truck and air controls more of your truck than you think, so air leaks can happen anywhere. Leaks can come from valves, air line hoses, the brake drum itself, diaphragms, your seat, etc. Having low air will cause the spring to stay up against the brake shoe and cause friction even when you aren’t pushing down on the brake pedal. That friction can cause your tires to start smoking or even catch on fire.

How Air Brakes Work

First, the air compressor pumps air into the air storage tanks, or reservoirs, that are stored beneath the truck. That air will then be pumped through the air brake lines where it will reach the air brake. With most typical “S-Cam” brake systems when you push the brake pedal on a CMV (commercial motor vehicle), air pressure pushes out a rod, moving the slack adjuster. The slack adjuster is used to calibrate the air brake system and to ensure that the internal spring mechanism is working appropriately, making sure the spring is not fully extended, which would not create the appropriate amount of friction needed to slow down the vehicle.

Air flows through the nozzle to the air brake chamber and that causes the spring to move the “S-Cam”. The “S-Cam” forces the brake shoe lining away from one another and presses them inside the brake drum. This creates the pressure and friction needed to slow the wheel down. Then when you release your foot from pressing down the brake pedal, the “S-Cam” rotates back, causing the spring to pull the brake shoes away from the brake drum getting rid of the friction and slow-down effect.

How to Check and Test Air Brakes

One of the most common issues with air brakes are air leaks, and the best way to tackle air leaks is to ward them off before they can occur through personal maintenance routines. Drivers can inspect their brake systems every day, even if you can’t get under their vehicles, you can listen for air leaks (a hissing sound that can be faint to loud), check low air signals, and look for component damage. If you are able to get under your vehicle, you can measure pushrod stroke the same way a CVSA certified inspector does and compare the results to the pushrod stroke limits set by regulation. Drivers can physically test the air brake system, but you can also look for line kinks, wear, and potential damage to the air system components. Keep in mind that this test will take a while to complete and for the air tanks to refill.

Here are steps you can take to test your air brakes:

  1. Shut off engine
  2. Release the YELLOW tractor protection valve/parking brake valve. RED valve should be in also
  3. Roll down window and listen for any audible air leaks which may be blown diaphragm in the chamber or leak in line
  4. Make a note of the air pressure
  5. Apply the foot break and hold for one minute
  6. Note air pressure drop resulting from brake application. Check the air gauge to see if the air pressure drops more than three pounds in one minute


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