How Do Air Brakes Work - Ultimate Truckers Guide

How Do Air Brakes Work – Ultimate Truckers Guide


When you have an Air brake system on your vehicle the failure mode is brake application. So, if you have a massive leak in the system, you won’t have a brake failure. Instead, your vehicle will stop and not move until you replace, repair or bypass the system.

Totally different scenario happen if you have hydraulic system, because a failure mode there mean no brakes and you can end up risking your life as well as other people lives. Best case scenario, you can end up damaging property, but again, I don’t even want to imagine the worst case scenario!

Source: Youtube

Furthermore, the hydraulic brake system in light vehicles requires a separate parking brake system usually including:

  • Steel;
  • Cables;
  • Some sort of application system of levers on the brake shoes or brake calipers; or
  • Separate system involving a driveline brake or a separate brake drum at the wheels.


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The good side of Air brakes is the fact that with the Air brakes system; the brakes have built in heavy springs. They will apply the brakes when the pressure fails. Furthermore, they are parking brakes also, and only need an air valve on the dash to control them.

I hope I was able to explain how do Air brakes work in this short paragraph, but in case you need more, read the next sub line.

How do Air Brakes Work with Large Trucks?

You probably wondered by now how do air brakes work with large trucks, and why large trucks are not using liquid hydraulic fluid instead.


Well, lighter types of vehicles such as cars use hydraulic brakes , while Air brakes are their alternative when it comes to heavy vehicles such as trucks.

You probably know all this, but let me repeat it! In order to stop the car, the Hydraulic brakes are using a hydraulic fluid to transfer pressure from the brake pedal to the brake shoe.


On the other side, the  large multi-trailer vehicles use Air brakes, and have quite few advantages in their favor, including:

  • The brake system can never run out of its operating fluid (because the supply of air is unlimited). The hydraulic brakes can any time run out of operating fluid and result in brake failure.
  • It is much easier to attach and detach airline couplings then hydraulic lines. You don’t have to worry about letting air into the hydraulic fluid. Furthermore, you don’t need special trained personnel, to attach and remove the circuits of trailers.

The air used for transmission of force, also stores potential energy that can serve to control the force applied for.

These systems have an air tank that stores enough energy to stop the vehicle in case the compressor suddenly stops working.

Air brake systems have enough “fail-safe” capacity. This way if the vehicle happens to have a minor leakage, brakes will still be effective and stop the vehicle safely.

How do Air Brakes Work?

Slowly but surely we’ve come to the final point of my article today, and answer the question how do Air brakes work!? I think I’ve cover a lot   on how do Air brakes work, but here one more time I will say everything that I forgot to mention earlier.

Source: Youtube

At its most basic level, the Air brake operates pretty much same like the piston in an internal combustion engine inside its cylinder. The Air storage tanks are filedl with compressed air by the compressor, which is powered by the engine.

Basically, if the driver steps on the brake pedal, or the treadle valve, the air from the storage tanks will flow into the cylinder, and push the piston down the cylinder also known as the brake chamber. This is the service portion of the system.

The sequence by which this works is: the piston is attached to the push rod, which turns the slack adjuster which then connects the push rod to another rod, and then it rotates.

Source: Youtube

This rod is connected to the S-cam.

We have several types of Air brakes, including S-cam and wedge type brakes.

The S-cam

These types of brakes are the most popular among the drivers. After all it is the S-cam that turns and pushes the brake shoes against the lining of the drum brake!

The semi-trailer’s Air brakes are connecting to a tractor with two lines. The first line is  the emergency line or the supply line.  Normally, it is bigger than the other and its ether red ether with red fittings. This line provides air pressure to fill the semi-trailer’s reservoir tank and the pistons that activate the brakes.

Source: Youtube

The second line called service line, its normally blue colored or with blue fittings, and it’s usually smaller than the other one.

When you are performing a normal breaking, pressing the brake pedal will pressurizes the service line. Whit that happening, the valve in the trailer is activating and will direct air from the reservoir and the emergency line to the brake cylinders.

With that the piston moves and activates the brakes.


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If you let the pedal up, it will decrease the service line pressure.

With the drop of the service line pressure, the valve in the trailer will block the air supply from the reservoir.

At the same time will release the pressure in the brake cylinder, and release the brakes.


So this is exactly how do Air brakes work! The system is something like a servo or amplifier.

If the pressure in the emergency line drops, because of the disconnection of the emergency line coupling, the activation of a valve in the cab, or a break in the emergency line, the spring brakes will activate immediately, because there is no air pressure in the trailer air tank left, to hold them back.


Here we came to the end of this ultimate truckers guide about how do Air brakes work!



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