How Do Air Brakes Work on a Truck?

If you are driving a big rig or any other vehicle that requires a lot of stopping power, it is crucial to understand how air brakes work. This type of brake system uses compressed air to slow or stop the vehicle and is commonly used on large trucks and buses.


The Advantages of Air Brakes

Air brakes can generate more stopping power than other types of brake systems. They work by applying pressure to the brake pads, which slows down or stops the vehicle. There are two types of air brakes: disc brakes and drum brakes. Disc brakes use a caliper to apply pressure to the brake pads, while drum brakes use a wheel cylinder to apply pressure to the brake shoes.

What Happens When You Press the Brake Pedal on Air Brakes?

When you press the brake pedal on air brakes, a pressure differential is created between the atmosphere and the chamber where the brake shoes reside. This differential forces the shoes apart, pressing them against the drum. The friction between the shoes and drum slows the wheel’s rotation and ultimately stops the vehicle. The pressure differential is created by a compressor that takes in atmospheric air and pumps it into the chamber.

Maintaining Air Brakes

The chamber must have enough air to create the necessary pressure differential for the system to work correctly. It is crucial to check the air level frequently and top off the tank as needed to ensure that your air brakes are always ready when needed.

Disadvantages of Air Brake Systems

One potential disadvantage of air brake systems is that they can be more expensive than other brakes. This is because air brakes compress air, which results in moisture. This moisture then requires an air dryer to remove it, which can increase the price of air brakes.

In addition, air brakes can also require more maintenance and repairs, particularly in the first five years. Despite these potential disadvantages, air brakes are still considered one of the most effective braking systems available.

Releasing and Engaging Air Brakes

Releasing the air brakes is a two-step process. First, the driver must release the parking brake by pushing on the knob. This supplies air to the air chambers, which push against the springs and release the parking brakes. Next, the driver must pull back on the handle to release the service brakes. This allows air to flow into the chambers and push against the shoes, which releases the brakes. If the pressure in the air system drops below 60 psi, the parking brake knob will pop out and set the parking brakes.

Using Air Brakes in Emergency Situations

While air brakes have many advantages, they can be challenging to use in an emergency. In an emergency, the driver should use the controlled or stab braking method to stop the vehicle. Air brakes require regular maintenance and can be expensive to repair.

Draining Air Tanks on a Truck

Truck air tanks must be drained regularly to prevent water buildup and rust. While it is possible to drain the tank manually daily, an automated system is recommended to avoid forgetting. Electronic drain valves are a good option for those who want to avoid having to remember to drain their tank manually. However, it is still essential to check the tank periodically to ensure it drains correctly. Failing to drain air tanks can lead to rusting and necessitate buying a new tank.


Air brakes represent a powerful alternative to hydraulic brakes, particularly for larger vehicles like buses and trucks. However, their benefits come at a price, as they require regular maintenance and can be expensive to repair. Additionally, it is crucial to understand how air brakes work to use them correctly in an emergency. Therefore, proper education and training are essential for anyone operating a vehicle with air brakes.

About the author, Laurence Perkins

Laurence Perkins is the passionate car enthusiast behind the blog My Auto Machine. With over a decade of experience in the automotive industry, Perkins has knowledge and experience with a wide range of car makes and models. His particular interests lie in performance and modification, and his blog covers these topics in-depth. In addition to his own blog, Perkins is a respected voice in the automotive community and writes for various automotive publications. His insights and opinions on cars are highly sought-after.