Why Do Trucks Use Air Brakes? The Most Detailed Answer

Why do trucks use air brakes is because they are safer and 100 percent reliable in heavy braking. It’s also easy to couple and uncouple a trailer to and from the truck without damaging the brakes because air can’t be contaminated.

Air Brakes on Trucks ~ Ran When Parked

There are many reasons why heavy vehicles use air systems for braking instead of hydraulics like their lighter counterparts. You’ll find all the details below, so keep reading.

Why Do Semi-Trailer Trucks Use Air Brakes?

Semi-trailer trucks use air brakes because they are safer and more reliable under heavy braking. It’s also quick to couple and uncouple a trailer without compromising the brakes because air can’t be contaminated. Unlike a hydraulic system, a leak doesn’t lead to a brake failure in air braking.

Air braking was initially invented by George Westinghouse, an invention that became a game changer in the railroad industry. With these brakes, trains could travel faster because they could stop more safely. Today, air braking is also used on trucks and trailers.

While these brakes have disadvantages (e.g., high initial cost), they are far better for heavy vehicles.

A loaded semi-trailer truck or semi-truck can weigh up to 40 tons (80,000 pounds). According to fundamental physics, the mass of a moving object is directly proportional to the force needed to stop the body.

In other words, as a vehicle’s weight increases, the braking force required to stop it also increases. Thus, semi trucks require a more reliable braking system, and that’s where air braking comes into play.

As mentioned, these brakes are used on trucks instead of hydraulic systems for many reasons. But the main reason is that breaking a heavy vehicle generates more heat than a smaller car.

As such, a hydraulic system won’t be 100 percent reliable in those conditions because the brake fluid can overheat, rendering the system inoperative. Moreover, the hydraulic brake components can sometimes lose pressure, causing the brakes to malfunction.

On the other hand, air braking system parts are impervious to heat – air can’t overheat and boil out of the system. Besides, air systems are designed to warn the driver when the air pressure drops (about 60 to 80 PSI) and engage the brakes when the pressure drops to extreme levels (20 to 40 PSI).

All trucks with air brakes have what’s called a pressure gauge on the pressure tank, which shows the amount of pressure remaining in the tank. Let’s dive deeper into why heavy commercial vehicles use air braking:

More Convenient When Coupling and Uncoupling a Trailer

Trucks use air braking because the systems are more practical when the vehicle needs to be repeatedly connected to and disconnected from a trailer. Also, there’s no need to bleed the brake liquid as the fluid is air.

That makes it more convenient when coupling and uncoupling from a trailer – some drivers do that several times a day. If you break an air hose and the pressure in the system gets low, the brakes apply by themselves.

Air Storage Tank ~ Ran When Parked

The air systems replenish themselves from the atmosphere, meaning the air storage tank is maintained automatically. Hydraulic fluid systems often capture some air as you hook up and disconnect the lines. It takes a tiny amount of air to render the system spongy or useless.

Moreover, the red fluid is sticky, meaning the unhooked fittings would collect dirt, causing contamination, leakage, and possible component wear.

With the air systems, leaking air damages nothing, and only water can contaminate the systems. And if any water enters the system, it’s easily removed by a filtering system and automatically emptied into the atmosphere each time the air compressor cycles off.

An Air Braking System Is Cheaper

An air braking system is simple and requires less maintenance than a hydraulic one. It also saves much time and labor during repair since experts only need to replace a leaking line or faulty part, and the brakes are good to go. The glad hands for the air systems are relatively cheaper than quick-disconnect hydraulic components.

Also, the blue and red lines are polarized to prevent misconnection. Besides, hydraulic brake systems work at a higher pressure than air systems. If the hydraulics were to be used to break heavy trucks, the various components would be more robust and presumably more expensive.

A Shorter Response Time

To understand this better, let’s discuss air brakes vs hydraulic brakes and how each works. How do air brakes work? They use compressed air and can be in the form of disc brakes, drum brakes, or both. An engine-mounted compressor pressurizes the air and then pumps it into the reservoir, storing the pressurized air until required.

Shorter Response Time ~ Ran When Parked

Note that drum-type brakes, for example, comprise multiple parts – slack adjusters, push rod, camshaft (S-cam), lining, shoes, drum, rollers, and return spring. When you press the brake pedal, the pressurized air (in the reservoir) enters the brake chambers.

Air pressure helps apply the service brakes and disengage the parking (emergency) brake – the pressurized air works to release the brake and not to use the force on the brake pads and drums. Multiple air circuits exist in the system.

The parking brakes are active by spring force within the brake section of the spring brake chamber once the air pressure in the chamber is released.

That allows the brake to also be used as emergency brakes. If pressure drops too low, the force the spring exerts in the chamber will overcome the force the air exerts on the diaphragm, applying the brakes on all wheels.

As with the hydraulic systems, air pressure is applied to the fluid when you press the brake pedal. The fluid multiplies the force of your foot and distributes it to the wheels to cause the car to slow or stop.

Considering the length and structure of a truck or other heavy-duty vehicle, the brake lines are relatively longer than cars. That’s due to the longer wheelbase.

If hydraulic brakes were to be used in such vehicles, following the hydraulic circuit and the fluid rushing in the brake calipers would take much time to reflect the braking action in the wheels. Air braking helps to avoid that delay by providing a quicker response time and minimizing the risk of an accident.

Air Systems are Easier To Use

Air can store energy at rest, but a hydraulic fluid doesn’t. In other words, the way hydraulic systems are designed, they hold no braking potential until pressure is applied. But in an air system, the air is already stored under pressure (usually around 125 PSI). The best part is the pressure in both the truck and the trailer.

Using Air Systems ~ Ran When Parked

That means that the pressure to apply brakes is available only a few feet away from the brakes. And with the compressed air supply close enough to the wheels, the only thing required to apply brakes is a signal pressure of your foot on the pedal.

To learn how to use air brakes on a truck for regular stops, push the pedal down, and control the pressure to make the vehicle come to a smooth and safe stop. For a car using a manual transmission, it’s recommended that you avoid pressing the clutch until the engine RPM comes down to almost idle.

Shift into a starting gear when stopped. In case of emergency braking, brake in a manner that will keep the truck in a straight line and enable you to turn if required. You can use the stab or controlled braking method. Stab braking should only be used on trucks without anti-lock systems.

Press the pedal to the floor and release it when the wheels lock up. When the wheels begin rolling, floor the brakes again. Reapplying the brakes before the wheels start rolling will cause the vehicle not to straighten out.

Controlled braking involves applying the brakes as aggressively as possible without locking the wheels. At the same time, you must keep the steering wheel movements small to none. If the wheels lock or require steering large, release the brakes, then reapply them immediately.

Leak Doesn’t Result in Brake Failure

In an air system, a massive leak doesn’t cause a brake failure. Instead, it results in the vehicle stopping. The failure mode is brake application and being unable to move until you bypass or repair the system. If a truck air brakes fail, it will likely stop in the middle of the road, blocking a traffic lane.

Result in Brake Failure ~ Ran When Parked

That’s far better than driving through someone’s house. But in a hydraulic system, the failure mode is an inability to stop (no brakes), which can be terrifying, especially for a heavy truck.

Also, unlike a hydraulic system that requires a separate brake (e-brake or handbrake), often consisting of steel cables or something similar, air braking is a two-in-one system.

It’s complete with the service system and the parking or emergency brake. The parking system works oppositely, as mentioned. Springs apply the brakes if the system lacks pressure, which is a perfect fail-safe system.

Insensitive to Temperature Changes

In hydraulic systems, the fluids can thicken when the system gets too cold, reducing the braking power. Conversely, when the system gets too hot, the fluid can expand, leading to braking issues.

Thus, semi trucks use air braking because, as stated earlier, air isn’t affected by these temperature variations like fluids. So, if you once asked yourself why do trucks use air brakes in the winter, now you’re informed.

Temperature Changes in Trucks ~ Ran When Parked

Note air brakes aircraft has been one of the most searched phrases online. If you also want to know, air braking systems are also used in many airplanes in flight, and when engaged, an air brake on the wings expands vertically.

The expansion reduces the aerodynamic properties of the aircraft’s wings. As the wings become less aerodynamic, the airplane is exposed to more significant drag, reducing speed.


You now have the answer to why do trucks use air brakes after going through the various reasons discussed in this article.

Let’s summarize some of them before you leave:

  • Semi-trucks and other heavy vehicles use air braking mainly because they are more reliable at heavy braking.
  • Unlike the hydraulic system’s parts and fluid, the air system’s components are impervious to temperature changes.
  • For example, if the hydraulic fluid is exposed to extreme temperatures, it loses its braking properties, rendering it less effective or (in the worst-case scenario) inoperative.
  • Another critical reason for using air braking systems on trucks is that they are more practical when a vehicle requires to be repeatedly disconnected from and reconnected to a trailer.
  • Other reasons include cheaper replacement parts, a shorter response time (which enhances safety), ease of use, and more reliable fail-safe features.

As stated, you’re now well-informed on this topic. Please feel free to refer to the numerous air brake system pdfs on the internet if you need further information on the brake systems.


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