How Do Air Brakes Work?

Air brakes are essential components that enable trucks to transport thousands of heavy pounds of weight safely. All heavy vehicles; trucks, busses, trailers’ braking systems consist of air brakes. But why are air brakes used on heavy trucks? Air is free and everywhere, but brake fluid is not; fluids can leak anywhere and cause brake failures.

Since accidents on heavy vehicles can cause catastrophic damages to lives and properties, heavy brakes are equipped with air brakes to eliminate fluid leaks and reduce the rate of accidents to the nearest minimum. Hence, bus drivers are expected to know how to use air brakes on buses and every other driver who drives an air brake-powered vehicle.

Due to the importance of air brake systems in trucks, buses, and trailers, diesel mechanics need to know air brake system components and locations. They should also know how to diagnose, service, and maintain it for optimum brake performance.

If you’re asking ‘how do air brakes work,’ this article has provided sufficient information on this subject matter and provides answers to users’ frequently asked questions. Sit back and have a 5minute read.

how do air brakes work on a truck

Why Do Trucks Use Air Brakes?

Do you hear a loud unmistakably hiss from a truck or buses on highways or major cities as they slow down or come to a complete halt? That loud hiss is normal. And I know you’ll be wondering, why are air brakes used on heavy vehicles since they make such a loud noise? Why didn’t the manufacturers eliminate these noises with brake fluids?

Those are pretty nice questions, but here are the reasons; availability and reliability. Big vehicles that carry tens of thousands of pounds of weight use air brakes because it is available everywhere and reliable.

The hydraulic brake system on small passenger cars and SUVs uses brake lines and fluid reservoirs. It needs to be filled manually with brake fluid in a service shop. Availability and reliability of air are one reason air brakes are common in big vehicles.

Air brakes are produced and configured so that inactive settings remain closed, which means that if the air pressure suddenly drops below 45 PSI, the system will automatically activate the brakes. The air brake systems are designed with strong springs around the pistons that pull out and stop all the wheels from moving when the driver steps on the gas pedal.

The wheels will remain in this position until a higher air pressure of about 65 PSI forces the pistons back into their place. This braking system is designed for all heavy trucks, trailers, and buses for a vital purpose. They are stronger at stopping cars and are safer than hydraulic brakes. Air brakes have a sweet simplicity; the driver can activate them by switching a knob on the dashboard. Once the driver activates the knob, it starts sending out air pressure which allows the springs to do their functions; slowing down the truck or putting it to a complete stop.

Aside from this, trucks are designed so that a single tractor can control multiple trailers. This means that a driver can activate all the trailer’s regular brakes with a push of a button. Suppose the tractor is out of the body, all the emergency brakes will automatically deactivate.

When servicing a hydraulic braking system, if there are leaks or you detached the lines from any joints, you will need to flush air from the brake lines by bleeding the wheels. Any air in the system will cause a soft or spongy brake pedal. In contrast, in air brakes, you don’t need to go the extra mile of bleeding or flushing the system. The technician will only need to replace the rusted or leaking lines.

Some trucks, buses, and trailers are equipped with dual brake systems, where the axle consists of different air tanks and airlines. It reduces the labor involved in fixing the brakes since issues with one air tank or line do not affect the others. However, the brake system is controlled by a single brake pedal.

Are there any disadvantages associated with air brakes? Yes, a common demerit of air brakes is water vapor build-ups in the brake lines. Water vapor build-ups can cause severe brake issues. It freezes during the winter season. This issue is common with those residing in snow areas.

In any case, you can prevent water vapor issues by using drains and dryers coupled with proper maintenance.

Whenever there’s water vapor in the storage tank or brake lines, it’ll take an extended time to slow down or bring a truck to a complete halt since it’s not transferring the required air effectively.

Another downside is that vehicles with air brakes are restricted to be driven by drivers with class 1 licenses or other license classes with Q endorsement, which calls for extra education, financial investment, and training before qualifying to drive air brakes vehicles.

Considering the availability of air and the reliability of air brakes, the extra financial investments, education, training, and water vapor build-ups are not truly disadvantaged.

how do air brakes work on a bus

How Do Air Brakes Work?

Unlike hydraulic brake systems, air brakes function with compressed air from the compressor, stored and transported via the storage tanks and the airlines instead of brake fluids. Heavy trucks, buses, and trailers are either equipped with brake drums or brake discs. However, some trucks come with both disc and drum brakes.

An installed engine compressor generates the air used in air brakes. After generating the air, the compressor transports it to the air tank. The air tank stores it until when it’s needed.

Applying the service brakes and releasing the parking or emergency brakes require air pressure. The system consists of multiple air circuits. When the driver releases the air pressure in the chamber, it engages the parking brakes with the force of the spring around the piston.

This allows the driver to use parking brakes as emergency brakes. If the air pressure goes down to around 45 PSI, the spring’s pushing force will get control of air pressure on the diaphragm and engage the brakes on all the wheels.

You may not be wrong to say that air brakes work like the hydraulic system. The difference is, when a driver steps the brake pedal on the air braking system, the system sends compressed air to all the wheels to stop the car. In contrast, the braking system on the hydraulic brake sends brake fluid to the wheel when a driver depresses the pedal.

How To Use Air Brakes

Air brakes are used for normal stops, emergency stops, controlled braking, and stab braking. Here’s how to use air brakes.

Normal stops

Like hydraulic braking systems, apply air brakes on normal stops, step on the brake pedal. You need to control the pressure to bring the vehicle to a safe and smooth halt. If you’re driving a manual transmission, do not depress the clutch until the engine RPM is low; close to the idling rate.

Emergency stops

If a semi-truck or small passenger car suddenly pulls in front of you, apply the brake immediately. However, that depends on the speed you’re traveling.

If you’re at high speed, you need to brake in a way that won’t wave your steering of the straight lane and allow you to steer off the obstacle.

Controlled braking

This method implies applying the brake as hard as possible while ensuring the wheels do not lock up. If you need to make a sharp turn, release the brake and re-engage it immediately.

Stab braking

This method is only used on vehicles with an anti-lock brake system. Apply the brake and release it as soon as the wheels lock up. Once the wheels start moving, reapply the brakes again.

Typically, it takes around 1 second for the wheels to start moving after releasing the brakes. Do not reapply the brakes before the wheels start rolling. If not, the wheels will not straighten out.


Q: What happens when air brakes fail?

With advances in technology, it is nearly impossible for air brakes to fail. They are more or less bulletproof.

However, if the air brake system has low air pressure, it’ll trigger an alarm that indicates low air pressure in the braking system. In any case, trucks, buses, trailers, trains, and airplanes are designed with backup emergency brakes that activate automatically when the air brake fails, making it impossible for vehicles with air brakes to fail.

Q: Are air brakes hard to use?

Air brakes are suitable and reliable braking systems for heavy vehicles with a gross weight of around 26,000 pounds and above. Twenty-six thousand pounds of weight will outweigh the hydraulic brake systems, which is why that is replaced with air brakes.

However, brake fluids cannot be compressed, and air can, it is very difficult to bring air brakes to a complete stop smoothly. When a driver applies the brake on a hydraulic braking system by stepping on the pedal, the same pressure is applied to all the wheels.

In contrast, when a driver applies a brake on the air braking system, it creates a pressure wave that travels to all the wheels simultaneously before stopping.

Q: How long should it take for a truck to build air?

Air compressors have a cut-in pressure of around 20psi or lower and a cut-out pressure of 110 psi and 130 psi.

In any case, air pressure from 85 to 100 psi will take less than a minute to build up.

Q: Which is better: air brakes or hydraulic brakes?

Air brakes are more or less bulletproof. They are better than hydraulic brakes. Using compressed air instead of brake fluids eliminates unnecessary brake failures.

For instance, any leak on brake lines will cause brake failures. On the other hand, minor or significant brake leaks on the air brake systems will not cause the system to fail.

Q: Why do air brakes take longer to stop?

The primary reason why air brakes take a longer time to stop is that it uses air. Air takes longer to travel to all the wheels than hydraulic.

When you depress the brake pedal on heavy vehicles with air brakes, it transports the compressed air from the storage tank to the wheels simultaneously. Hydraulics takes a shorter time compared to air.

Q: Why are disc brakes not used in trucks?

Disc brakes are still used in trucks. However, 95 percent of trucks in the US use drum brakes.

The significant reason trucks rely on air brakes is that they require less replacement than disc brakes. And are cost-effective. Since they require less maintenance, they are a better option than disc brakes.

Final words

At this juncture, we have explained how to use air brakes on trucks and other heavy vehicles.

If you have followed this post religiously to this point, you’ll no longer ask, ‘how do air brakes work.’ So when next you hear a loud hiss from a truck or bus behind you on the road, you won’t fret knowing they are reliable brake systems and are built to make such noise.

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Osuagwu Solomon

Osuagwu Solomon is a seasoned automotive technician for the past 9 years, and a technical writer. He loves writing about auto professional repair guides, DIY repair guides, and buyer’s guide. After spending six years in the automotive workshop, he decided to impact his knowledge to people aside his domain, and he has achieved this by centering his Automotive writing skills on REPAIRS.

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