How Do Air Brakes Work?

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 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 brake systems function 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 how do air brakes work, which is covered in the UTI diesel program.

How Truck Air Brakes Work

Air brakes 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, when the driver presses the brake pedal, air pressure is applied, like hydraulic pressure in a hydraulic brake circuit to the wheel when applying the brake.

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.

Learn How Air Brakes Work Yourself

If that sounds complicated, don’t worry. UTI’s diesel 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.

“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,” says Brian. “This allows the students to hit the ground running once they get into the workforce.”

Request information on UTI’s diesel program online or call 1-800-834-7308 for information.