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When drivers are transporting tens of thousands of pounds of weight in a big diesel truck, one of the most important components of that vehicle is the braking system, to ensure that the vehicle can come to a stop.

Big vehicles including buses, transportation trucks, and tractor-trailers have a braking system that typically consists of air brakes as opposed to hydraulic fluid. There is an abundance of air available, while hydraulic fluid could get lost in a leak, therefore, big vehicles rely on air-powered brakes to keep drivers and everyone else safe on the road.

This article will provide you with all the relevant information on air brakes and how do air over hydraulic brakes work!

Table of Contents

How Air Brakes Came About

What is airbrake? Trains, buses, and tractor-trailers use an air brake system because air is everywhere and hydraulic fluid isn’t. Relying on hydraulic fluid could be a massive mistake in the event of a leak, as it will run out. If a heavy passenger or cargo load vehicle like the ones listed relied on hydraulic brakes, it could become deadly if the brake system suddenly busted a leak.

Before air brakes were around, you’d often see an operator, or brakeman, handling the primitive brake systems that trains used in each car to apply the hand brake at the signal of the engineer or train director.

These highly inefficient manual systems were soon to be replaced by the direct air brake system which uses an air compressor to feed air through a brake pipe and into the air tanks on each car. Now when the engineer applies the brakes, the pipes fill with air and squeeze the brakes.

Understanding Air Brakes & How Do Air Brakes Work?

Air brakes work by using compressed air instead of hydraulic fluid. Air is pressurized by an engine-mounted compressor which then pumps the air into the air tank receivers, to store the compressor until it’s needed.

Air pressure is then used to apply the service brakes and release the parking brake. There tend to be multiple air circuits in the system on any vehicle with air brake systems. How do air brakes work on a truck? When the air pressure in the chamber is released the parking brake engages by spring force in the parking brake section of the spring brake chamber.

This then allows for the parking brake to be used as the emergency brake system if it is required. How do air brakes work on a semi? The trick is that if the air pressure was too low, the force exerted by the air on the diaphragm will be overpowered by the force exerted by the spring in the chamber which will apply the brakes on the wheels.

How do air over hydraulic brakes work? You may think of air brakes as working similarly to a hydraulic brake circuit. Just like 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.

Air brakes have an unlimited supply of air and can still operate during minor leaks. On the road, air brakes are often used on larger trucks and heavy commercial vehicles. When the brake pedal is pressed, air travels through the air brake chambers, causing a mechanism to apply pressure to the drums or calipers to help to slow the vehicle down or bring it to a complete stop.

Air pressure will actively hold back large springs that enable the emergency brake. If the air brake system loses pressure due to an air leak or disconnect, the air brakes will automatically engage. As you then step on the brake pedal, pressure in the system is released, which will deploy the brakes gradually.

This, therefore, requires the compressor to build up more pressure, so that the brakes are held in the open position. Most modern-day air brake systems come equipped with alerts to let you know if you’re losing too much pressure.

Types of Air Brakes

Air brakes can be either drum brakes or disc brakes, or possibly even a combination of both. We’ll take a look at how each works:

How Drum Air Brakes Work

Here’s how a drum brake operation works:

  1. The operator depresses the brake pedal, which actuates the air braking system.
  2. The air from the reservoirs is supplied to the brake valves.
  3. The air from the brake valves is then delivered to the brake chambers.
  4. The chambers move the pushrod to push on the slack adjusters.
  5. The adjusters transfer the pushrod force into a cam rotational force.
  6. The cam rotates which causes the rollers to rise and force the shoes against the drum.
  7. The shoe linings then make contact with the drum to slow or stop the wheel.
  8. The operator releases the brake pedal and the delivered air exhausts.
  9. The brake shoe return springs force the shoes to release contact from the drum and return to their original position, while the cam also rotaes back to its original position.
  10. The slack adjuster returns to its original position and the brakes are released.

How Disc Air Brakes Work

Here’s how a disc brake operation works:

  1. The operator depresses the brake pedal, which actuates the air braking system.
  2. The air from the reservoirs is supplied to the brake valves.
  3. The air from the brake valves is then delivered to the brake chambers.
  4. The brake 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 makes contact with the brake rotor.
  6. A bridge moves with the caliper to move the outer pad against the rotor.
  7. The pads then squeeze against the rotor, transferring force to slow or stop the wheel.
  8. The operatorreleases the brake pedal.
  9. The return spring forces the caliper and bridge back to its rest position.
  10. The brake pads separate from the brake disc and the brakes are released.

Air Brake Components

“Foundation air brakes” are the most commonly used air brake system that can be found in trucks and buses, and essentially work in the same way as in rail cars. They all use the triple-valve principle, where air builds up inside the brake pipes or airlines and releases the brakes.

Almost all of the road vehicles that are equipped with air brakes have a graduated release system, where a partial increase in pressure dictates a proportional release in brakes. The following components are exclusive to a foundation air-brake system in trucks or buses:

  • Air compressor
  • Air compressor governor
  • Air reservoir tanks
  • Brake chambers
  • Brake S-cam
  • Brake shoe
  • Drain valves
  • Foot valve (brake pedal)
  • Push rod
  • Return spring
  • Slack adjusters

Air Compressor

The air compressor plays the role of compressing atmospheric air and pumping it into the storage tanks to be used in the air brake system.

Air Compressor Governor

The air compressor governor controls the cut-in and cut-out point of the air compressor to maintain a set amount of air in the tank or tanks. Often referred to as the air compressor pressure switch, you can learn more about them here!

Air Reservoir Tanks

Air reservoir tanks hold the pressurized air to be used by the braking system. Visit our Compressed Air Tanks Explained to learn more about them.

Brake Chambers

The brake chambers are cylindrical containers that house a slack adjuster that moves a diaphragm or cam mechanism.

Brake S-Cam

An s-shaped brake cam is what pushes brake shoes apart and against the brake drum.

Brake Shoe

The brake shoe is a steel mechanism with a lining that causes friction against the brake drum.

Drain Valves

Drain valves are what release and drain the air in the tanks when the vehicle isn’t in use. Read our How To Drain Water From Any Air Compressor Tank – Remove Water From Air Tank guide for more information!

Foot Valve (Brake Pedal)

When the foot valve is depressed, the air is released from the reservoir tanks.

Push Rod

The steel push rod, similar to a piston rod, connects the brake chamber to the slack adjuster. When depressed, the breaks are released, and if extended, the brakes are applied.

Return Spring

A stiff return spring is connected to each of the brake shoes that return the shoes to the open position when not spread by the s-cam or the diaphragm.

Slack Adjusters

The slack adjuster arm connects the pushrod to the brake s-cam to help adjust the distance between the brake shoes.

You may have wondered why trucks and buses make those unusual squeaking and hissing sounds? The squeaking is the air escaping after the vehicle has braked and the psssh sound is the automatic bypass safety valves at work, ensuring that the air pressure remains at the correct level.

Since the main advantage of air brake systems is their ability to use air to operate, the compressor is constantly kicking on and kicking off to refill the reservoirs with pressurized air. When the compressor builds too much air, the valves open, producing that loud hissing noise.

Air Brake Preventative Maintenance

Each and every state in the U.S. will have specific guidelines for how to operate a vehicle with air brakes. Not only is the test to obtain a commercial driver’s license demanding, but so are the steps to maintain such a vehicle. Here are preventative maintenance steps you’ll want to take before heading out on the road:

  • You must make sure the minimum operating pressure for a vehicle air brake system is no less than 85 for a bus and 100 psi for a truck.
  • Be sure to check that it takes no longer than two minutes for air pressure to rise from 85 psi to 100 psi at 600 to 900 rpm. Commonly referred to as the air pressure build rate.
  • Confirm that the correct cut-out governor pressure for the air compressor is between 120 psi and 135 psi and the cut-in pressure is 20 psi to 25 psi below the specified cut-out pressure.

PSI stands for pounds per square inch of pressure, to learn more, visit our PSIA vs PSIG vs PSI – Differences, Conversions & Calculations guide!

You’ll also want to watch for the potential water in the air brake system, a natural byproduct of the condensed air. In colder climates, this water can turn to ice, which will block air from reaching the brake mechanism and naturally cause the wheel to lock up. To prevent this problem, many of the modern systems have automatic drain valves installed in each air tank to drain the condensation and moisture build-up from the system.

Air couplers may also pose a problem. If they have worn rubber seals then this will cause air to escape. While the compressor can overcome small leaks, running compressors too hard can lead to premature wear and ultimately failure. Air loss isn’t necessarily a bad thing, but it will mean the vehicle may become stuck.

FAQs (Frequently Asked Questions)

Why do big trucks use air brakes?

Air brakes are used on big trucks/semi-truck instead of hydraulic brake fluid because these trucks typically carry a lot of weight. Compressed air can be constantly produced, unlike hydraulic fluid, which requires refills and is more likely to leak, leading to brake failure and potential accidents.

Why are air brakes not used in cars?

Most cars use disk brakes which air pressure would not be sufficient to operate. On top of this, air brakes freeze up in winter and colder conditions, and on such small vehicles, this can become a serious drawback. Most cars have 4 wheel disc brakes and despite being problematic and prone to warping, they have reached maximum performance in regards to stopping distance.

Do air brakes run out of air?

The whole idea behind air brakes is that the supply of air is unlimited and they can never run out of their operating medium as hydraulic breaks do. Even with minor leaks to the brake system, air brakes will not result in brake failures.

Is driving with air brakes different?

What fools new truck drivers is the fact that the air brake systems do not automatically release like hydraulic brakes, depressing the brake pedal forces hydraulic fluid into the brakes; releasing the pedal forces fluid back out. Air brakes are different than this as they remain deployed until the system starts to recharge.


If you have any questions about air brakes, please leave a comment below, with a photo if applicable, so that someone can help you!