How Air Brakes Function

Picture this: It's your first week on the job as a dockhand at a shabby trucking company. The place is buzzing with activity as everyone rushes to load the final pallet of cargo onto a massive tractor-trailer headed cross-country. Suddenly, a foreman tells you to move one of the trucks so another driver can back into the loading dock. Assuming you’re capable of driving one, the foreman carries on, but you freeze – because you’re not.

Eager to impress the higher-ups and ignoring the fact that you lack a truck driver’s license, you jump into the driver’s seat, close the door, and turn the key. Just before the diesel engine roars to life, you’re hit by an ear-piercing alarm and a flashing dashboard light. You start the engine, but the sound and light won’t stop.

Having driven a stick shift before, you think you know what you're doing. Despite the chaos around you, you press the clutch, grab what you assume is the low gear, and slowly release the clutch. Instead of moving forward as you expect, there's a loud bang, the engine stalls, and you’re nearly thrown out of the cab.

You try restarting the engine, thinking you might have put the truck in the wrong gear, and select what you believe is the correct one. Yet, the buzzer and flashing light continue to wreak havoc in the cab. Maybe the emergency brake is still engaged. Unable to find any familiar brake handle or lever like you’d see in a car, you decide to release the clutch and give it another attempt.

To your embarrassment, the same issue occurs again. Out of the corner of your eye, you spot that same foreman yelling at you from the loading dock. Frustrated, you leap out of the cab, raise your hands in confusion, and watch as the annoyed supervisor rushes toward you.

Welcome to the world of air brakes. In this article, you’ll discover how air brakes and their components function, how to maintain an air-brake system, and why you couldn’t move that truck. Now, let’s dive into how George Westinghouse played a part in getting you into this mess.

George Westinghouse and the History of Air Brakes

Air is everywhere, but hydraulic fluid isn’t. Trains, buses, and tractor-trailers use air-brake systems to avoid depending on hydraulic fluid in car braking systems, which can run out if there’s a leak. These vehicles carry heavy loads of passengers or cargo, making safety a top priority. A high-speed locomotive relying on hydraulic brakes would turn into a lethal steel projectile if its brake system failed due to a fluid leak.

Before air brakes, trains relied on a basic braking system that required a brakeman in each car to manually apply a handbrake when signaled by the train's engineer or director. This labor-intensive and inefficient method was eventually replaced by direct air-brake systems, which used an air compressor to send air through a brake pipe into air tanks on each car. When the engineer activated the brakes, air would flow into the pipe, engaging the brakes.

In 1869, an engineer named George Westinghouse recognized the need for improved safety in the emerging railroad industry and invented the first triple-valve air-brake system for use on railcars. Westinghouse’s system worked differently from direct air-brake systems. The triple-valve system, as the name suggests, performed three key functions. Let’s break down those functions.

Instead of using force or directed air like hydraulic fluid in cars, the triple-valve system fills a supply tank and relies on air pressure to release the brakes. Essentially, the brakes stay fully engaged until air is pumped throughout the system. It's a clever design—if the system loses all air, the brakes automatically engage to stop the train. Now think about this when you’re driving down the highway and you press the brake pedal: If your car’s brake fluid leaked out, your brakes wouldn’t function.

The triple-valve system is the foundation of today’s air-brake systems found in trains, buses, and tractor-trailers. Let’s shift focus now and explore how air brakes function in road vehicles in the next section.

Understanding How Brakes Work

Before diving into air brakes used in road vehicles, let’s first examine how the brakes in your car function. Anyone who has driven a car knows that when you press the brake pedal, the car slows down and eventually comes to a stop. But how can we possibly stop a 3,000-pound (1,361-kg) car moving at high speeds with just our foot on a pedal?

To begin, let’s explore the various types of brake systems and their components. Every moving vehicle, including trains, trucks, buses, and cars, uses one of two systems. Hydraulic brakes, found in light-duty trucks and passenger cars, rely on hydraulic fluid or oil to operate. Air brakes, which we’ll explore in detail in the next section, use air to activate their braking mechanism. Let’s compare the two systems.

In a hydraulic brake system, fluid is stored in a reservoir called the master cylinder. When you press the brake pedal, the fluid is pushed through hoses or lines into pistons mounted on each wheel. These pistons then either press against brake shoes, which expand and create friction inside a brake drum, or they push against a brake pad that clamps onto a brake rotor. Below are the key components of a hydraulic disc brake system.

[source: Brakes]

Here’s a look at how some of the components work together in a disc brake system.

Before disc brakes, vehicles relied on drum brakes. The basic mechanics were similar, but drum brakes used brake shoes inside a drum mounted on the hub, whereas disc brakes use a rotor. Disc brakes provide better stopping power, as they cool more efficiently and offer a larger surface area for grip. Additionally, they vent brake dust, which forms as the brake pads wear down, much more effectively than drum brakes. For more details on the mechanics of disc and drum brakes, check out How Disc Brakes Work and How Drum Brakes Work.

Now that we've covered the basics of brakes in cars and trains, let’s shift our focus to large trucks and buses.

Components of Air Brakes in Trucks and Buses

Foundation brakes, which are the most common air-brake systems used in trucks and buses, operate in a manner similar to air brakes in rail cars. Utilizing the triple-valve principle, air accumulates in the brake lines, releasing the brakes. Nearly all road vehicles with air brakes are equipped with a graduated release system, where a minor increase in air pressure leads to a proportional release of the brakes.

Here are the components exclusive to a foundation air-brake system in trucks and buses:

When the vehicle is idle (foot off the brake and air system fully charged), air pressure holds the diaphragm or keeps the s-cam in the closed position, resulting in the brakes being disengaged. Upon pressing the brake pedal, the air pressure drops, causing the s-cam to rotate and push the brake shoes against the drum. The air compressor then refills the reservoir tanks, and when you release the pedal, air pressure returns to its original level, releasing the brakes.

Emergency air brakes serve as an additional safety feature to the standard air-brake system and are activated by pulling a button located on the dash (near the light button we discussed earlier). Before operating a vehicle with air brakes, you must engage the emergency brake by pressing the button to fill the system with air. The emergency brake will stay disengaged as long as the system remains pressurized. However, if there's a leak causing a loss of pressure, the emergency brake will engage automatically. Additionally, many heavy trucks are equipped with an exhaust brake, which helps slow the vehicle down but relies on the engine, not the air-brake system.

We’ve covered how air brakes function. Next, let’s explore how proper maintenance can prevent brake failure in the upcoming section.

Air Brakes: Preventative Maintenance

Each state in the U.S. has its own set of regulations for operating vehicles equipped with air brakes. Obtaining a commercial driver’s license requires passing rigorous tests, and maintaining such a vehicle involves several essential steps. Before hitting the road, here are a few important actions to consider:

It’s also important to check for water in the air-brake system, which results from condensed air. Water can be harmful to air-brake lines, especially in colder environments where it can freeze and block air flow to the brake mechanism, causing the wheels to lock. Many modern systems include automatic drain valves in each air tank to mitigate this issue.

Air couplers can present issues as well. Worn-out rubber seals may lead to air leaks. Although the compressor can manage a small leak, overworking it can cause failure. As we’ve learned, a loss of air pressure isn’t always detrimental, but it will leave you stranded. For truck drivers, finding yourself stuck on a mountain pass is definitely not part of the plan.

Brake sensitivity, a common feature of air-brake systems, can be a hazard, especially for drivers who are still gaining experience. These systems are specifically designed for vehicles that carry heavy loads. Have you ever wondered why you see multiple skid marks on the expressway? That's often due to light or empty trailers locking up their rear wheels. One of the biggest fears for truck drivers is jackknifing, which occurs when the back of the trailer swings up next to the cab. Trucks traveling through rain and snow are particularly vulnerable to jackknifing if too much braking force is applied.

Most modern air-braked vehicles are equipped with a dual system, meaning that they have two independent brake systems in case one fails. Tractor-trailers, for example, now come with Anti-lock brakes (ABS), which function in a similar way to the ABS systems found in passenger vehicles.

In principle, air brakes are both efficient and dependable. However, don't expect to find them in your car anytime soon. Air brake systems require significant space and maintenance, making them impractical for passenger vehicles. Just take a look at a Peterbilt truck cruising down the highway. Have you noticed the large tanks behind the fuel tanks? Now, imagine trying to fit those under the hood of a Honda Civic.

Air Brake Diagram

Let's now combine the individual parts to see how air brakes function as a complete system. This diagram offers both a detailed close-up and an illustration showing the placement of the brakes within your vehicle.