How Do Air Brakes Work? Air Brakes Explained Simply


Understanding Air Brake Systems: Enhancing Vehicle Safety

When drivers are transporting tens of thousands of pounds of weight in a diesel truck, one of the most important components of that vehicle is the brakes. In big diesel trucks, buses and tractor-trailers, the braking system consists of air brakes. Since air is plentiful but hydraulic brake fluid could get lost in a leak, big vehicles rely on air-powered brakes to keep drivers and everyone else on the road safe.

Because they’re a critical component of diesel vehicles, diesel technicians need to know how air brakes work and how to troubleshoot, service and maintain air brake systems.

How does an air brake system work? Brian Murphy, Universal Technical Institute (UTI) Education & Development Program Manager, Curriculum, helps answer the question, “How do air brakes work?” which is covered in the UTI Diesel program.

How Truck Air Brakes Work

Air brakes on trucks work using compressed air instead of hydraulic fluid. Air brakes can be either drum brakes or disc brakes, or a combination of both.

Air is pressurized by an engine-mounted compressor. The air compressor then pumps the air into the air storage tanks, which store the compressed air until it’s needed.

Air pressure is used to apply the service brakes and release the parking brake. There are multiple air circuits in the system. The parking brake engages by spring force in the parking brake portion of the spring brake chamber when the air pressure in the chamber is released.

This also allows the parking brake to be used as the emergency brake system. If air pressure was to drop too low, the force exerted by the spring in the chamber will be able to overcome the force exerted by the air on the diaphragm and apply the brakes on all wheels.

You might think of air brakes as working similarly to a hydraulic brake circuit. As with hydraulic brakes, air pressure is applied when the driver presses the brake pedal.

How Drum Brakes Work

Here’s how a drum brake operation works.

  1. The operator depresses the brake pedal, actuating the air braking system.
  2. The air from the reservoirs is supplied for the brake valves.
  3. The air from the valves is delivered to the brake chambers.
  4. The chambers move the pushrod to push on the slack adjusters.
  5. The adjusters transfer pushrod force into cam rotational force.
  6. The cam rotates, causing the rollers to rise and force the shoes against the drum.
  7. The shoe linings contact the drum to slow or stop the wheel.
  8. The operator releases the brake pedal.
  9. The delivered air exhausts.
  10. The brake shoe return springs force the shoes to release contact from the drum.
  11. The brake shoes return to their original positions, causing the cam to rotate back to its original position.
  12. The slack adjuster returns to its original position.
  13. The brakes are released.

How Disc Brakes Work

For disc brake operation, the process is slightly different.

  1. The operator depresses the brake pedal, actuating the air braking system.
  2. The air from the reservoirs is supplied to the brake valves.
  3. The air from the valves is delivered to the brake chambers.
  4. The chamber actuates the caliper, which transfers force to the inner brake pad.
  5. The caliper slides on the guide pins as the inner brake pad contacts the brake rotor.
  6. A bridge moves with the caliper to move the outer pad against the rotor.
  7. The pads squeeze against the rotor, transferring force to stop the wheel.
  8. The operator releases the brake pedal.
  9. The return spring forces the caliper/bridge back to its rest position.
  10. The brake pads separate from the brake disc.
  11. The brakes are released.

Components of an Air Brake System

An air brake system in vehicles comprises several essential components working cohesively to ensure safe and efficient braking.

The system includes an air compressor responsible for pressurizing air, which is stored in the reservoir tanks. These tanks store compressed air until needed for braking.

A governor controls the compressor, regulating the air pressure within specified limits. When the brake pedal is depressed, the air pressure releases from the reservoir through a series of valves and hoses, reaching the brake chambers or brake calipers.

The brake chambers convert the air pressure into mechanical force, activating the brake shoes or pads against the drum or rotor, consequently slowing down or stopping the vehicle. A quick-release valve allows for rapid release of air pressure, facilitating swift disengagement of the brakes.

The overall functionality and safety of the air brake system relies on the proper maintenance and synchronization of these parts.

Air Brakes in Different Types of Vehicles

Air Brakes in Cars

Air brake systems, while commonly associated with larger commercial vehicles like trucks and buses, are not typically found in standard passenger cars. Instead, passenger cars predominantly utilize hydraulic brake systems. These systems operate using brake fluid to transfer force from the brake pedal to the brake pads or shoes, creating friction to slow or stop the vehicle.

Air Brakes in Buses

When it comes to the safety and functionality of buses, especially those of substantial size and weight, air brakes are extremely important. Buses, similar to heavy trucks, commonly utilize air brake systems due to their efficiency in managing the significant braking demands posed by these vehicles.

Air Brakes in Trucks

Integral to the safe operation of trucks, air brakes play a crucial role in managing the substantial weight and stopping power required for these heavy vehicles. Trucks commonly employ air brake systems due to their ability to handle the demanding braking needs imposed by their size and load capacities.

Air Brakes in Semitrucks

Air brake systems serve as the backbone of safety and control in semitrucks, providing the necessary braking force to manage their immense size and weight. Semitrucks extensively rely on air brake systems, which are vital for ensuring the safety of operations, particularly when navigating highways while hauling heavy loads.

Air Brake FAQs

How do air brakes work in a vehicle?

As discussed earlier, air brakes operate using compressed air to control and facilitate the braking process in heavy vehicles. The system comprises several key components, including an air compressor, reservoir tanks, valves, hoses, brake chambers and brake shoes or pads.

When the driver presses the brake pedal, compressed air stored in the reservoir tanks is released through valves and hoses to the brake chambers. This release of air pressure activates the brakes, causing the brake shoes or pads to engage with the drums or rotors, creating friction and slowing down the vehicle.

What is the purpose of air brakes in trucks?

Air brakes serve a critical purpose in heavy vehicles, such as trucks, buses and large commercial vehicles, by providing reliable and efficient braking mechanisms tailored to handle their substantial weight and demanding braking requirements. Unlike hydraulic systems found in standard passenger cars, air brake systems use compressed air as the medium to transmit force, making them well-suited for heavier vehicles.

Learn How Air Brakes Work Yourself

If that sounds complicated, don’t worry. UTI’s Diesel Technology program has an entire course on Diesel Truck Brake Systems & Chassis. Students learn how truck brakes operate and the proper service and maintenance of the components.

You don’t have to have any prior experience to master subjects like air brakes at UTI. There are many graduates who entered the diesel program as complete beginners and leave ready to tackle entry-level diesel technician jobs.1

“UTI will expose the students to manufacturer’s tools, manuals and diagnostic software, as well as provide hands-on experience on diesel equipment such as full-size trucks and running engines,” Brian says.18 “This allows the students to hit the ground running once they get into the workforce.”

To learn more about this program as well as UTI’s other specialized training programs, simply request information. One of our Admissions Representatives can help you get on track toward pursuing a career in the industry! 

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1 ) UTI is an educational institution and cannot guarantee employment or salary.
2 ) For program outcome information and other disclosures, visit
18 ) UTI now offers all of its automotive, diesel, motorcycle and marine technician training in a blended-learning format consisting of online lecture courses along with in-person, hands-on lab training.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.


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