How Air Brakes Work

Image Gallery: Brakes Think you could move one of these bad boys in a pinch? See more brake pictures.
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Imagine it's your first week on the job as a dockhand at a rundown trucking company. Everyone is running around trying to finish loading the last pallet of cargo on the back of a huge tractor-trailer truck bound for the opposite coast. Suddenly, one of the foremen tells you to move one of the trucks out of the way so another driver can back up to the loading dock. Assuming you know how to drive such a vehicle, the foreman keeps going, but you pause -- because you don't.

Trying to please the higher-ups and to ignore the fact that you don't have a truck driver's license, you hop into the cab, close the door and turn the key. Before the diesel engine cranks, you’re startled by a mind-numbing buzzer and flashing light on the dashboard. You fire the engine, but the buzzer and light keep grabbing your attention.

You’ve driven a stick shift before, so you think you have it covered. Despite the sensory overload, you push in the clutch, grab what you think is the low gear and ease out the clutch. Instead of lurching forward as you expect, you’re greeted with a violent bang, the engine dies and you’re almost thrown through the windshield.

You restart the engine, figuring you put the truck in the wrong gear, and select what you think is the right one. Still, the buzzer and light cause havoc inside the cab. Maybe the emergency brake is still on. You don’t see any brake handle or lever that you would normally see in a car, so you decide to just let the clutch out and give it another shot.

Much to your embarrassment, the same thing happens. Out of the corner of your eye you see that same foreman hollering at you from the loading dock. Frustrated, you jump out of the cab and throw up your hands in bewilderment, as the scowling supervisor jogs toward you.

Welcome to the world of air brakes. In this article, you’ll learn how air brakes and their components work, how to maintain an air-brake system and why you couldn’t move that truck. Next, let's see how George Westinghouse got you into this situation.

George Westinghouse and Air-brake History

Air is everywhere. Hydraulic fluid isn't. Trains, buses and tractor-trailers use air-brake systems so they don’t have to rely on the hydraulic fluid in car braking systems, which can run out in the event of a leak. All of these types of transportation are weighed down by heavy passenger or cargo loads, so safety is of the utmost importance. A speeding locomotive that relied on hydraulic brakes would turn into a deadly steel bullet if the brake system suddenly busted a leak.

Before air brakes, trains used a primitive brake system that required an operator, or brakeman, in each car to apply a hand brake at the signal of the train director or engineer. This inefficient manual system was replaced by direct air-brake systems, which used an air compressor to feed air through a brake pipe into air tanks on each car. When the engineer applied these brakes, the pipe filled with air and squeezed the brakes.

In 1869, an engineer named George Westinghouse realized the importance of safety in the relatively new railroad industry and invented the first triple-valve air-brake system for railcar use. Westinghouse’s system worked the opposite way of a direct air-brake system. The triple-valve system performed three functions, thus its name. Let’s take a look at those functions.

  1. Charging: The system must be pressurized with air before the brakes will release. At rest, the brakes remain engaged. Once the system reaches its operating pressure, the brakes are freed and ready to use.
  2. Applying: As the brakes are applied, air pressure decreases. As the amount of air decreases, the valve allows air back into the reservoir tanks, while the brakes move to the applied position.
  3. Releasing: Once the brakes are applied and the air escapes after braking, the increased pressure releases the brakes.

Instead of using force or directed air to apply the brakes much like hydraulic fluid in our cars, the triple-valve system fills a supply tank and uses air pressure to release the brakes. In other words, the brakes in a triple-valve system remain fully engaged until air is pumped throughout the system. Pretty ingenious, considering if this type of system had a complete loss of air, the brakes would engage and stop the train. Think about that when you are zooming down the freeway and you hit the brake pedal. If your car’s brake fluid leaked out, your brakes wouldn’t work.

The triple-valve system is the basic concept at work in today’s air-brake systems in trains, buses and tractor-trailers. Let’s switch gears and learn how air brakes in roadgoing vehicles work in the next section

Understanding Brakes

Before we learn about air brakes in road vehicles, let's look at how the brakes in your car work. Anyone who has driven a car knows when he or she pushes the brake pedal towards the floor the car slows and eventually stops. But how in the world can our foot stop a 3,000-pound (1,361-kg) car traveling down the road at high speeds?

To begin with, let's discuss the different types of brakes and then we can explore the different components. Every rolling vehicle, including trains, tractor-trailer trucks, buses and cars contain one of two types of systems. Hydraulic brakes, found in light-duty trucks and passenger cars, use hydraulic fluid or oil to operate their brakes. Air brakes, which we'll break down in the next section, use air to operate their brakes. Let's look at the differences.

In a hydraulic system, fluid is stored in a reservoir commonly referred to as a master cylinder. When you push the brake pedal, fluid is pumped through brake hoses or lines into pistons mounted on each wheel. These brake pistons either push against two brake shoes, which expand and cause friction inside a brake drum, or against a brake pad, which clamps down on a brake rotor. Below are the components in a hydraulic disc brake system.

  • Brake reservoir: Contains hydraulic brake fluid
  • Master cylinder: Device that pumps the fluid from the reservoir to brake lines that run throughout the vehicle
  • Brake lines: Rubber or steel braided hoses that run from the master cylinder to each brake caliper
  • Brake caliper: A steel housing that mounts on a fixed point of the brake rotor that contains a piston and brake pads
  • Brake piston: A round rod that extends and pushes against a brake pad when hydraulic fluid is fed from the master cylinder
  • Brake pad: A metal backing pad with a semi-metallic overlay that grips the steel rotor
  • Brake rotor: A steel disc mounted to each wheel and hub that the pads grasp to stop the wheels from rotating

[source: Brakes]

Here's a look at how some of the parts fit within a disc brake.

Before disc brakes, cars relied on drum brakes. The principal mechanics were the same, but drum brakes used brake shoes set inside a drum that was mounted on the hub, versus a rotor. Disc brakes increase stopping power, as they are more easily cooled and have more surface area to grasp. In addition, brake dust, which forms as the brake pads wear and decreases braking ability, is vented more easily with disc brakes than with drum brakes. For more information on disc brakes and drum brakes, read How Disc Brakes Work and How Drum Brakes Work.

Now that we understand the fundamentals of brakes in trains and cars, let's talk about the big rigs and the buses.

Air-brake Components in Trucks and Buses

Diagram of air-brake components
Diagram of air-brake components

Foundation brakes are the most common air-brake systems found in trucks and buses and work the same way as in rail cars. Using the triple-valve principle, air builds up inside the brake pipes or air lines, releasing the brakes. Virtually all of the roadgoing vehicles 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 a truck or a bus:

  • Air compressor: Pumps the air into storage tanks to be used in the brake system
  • 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
  • Air reservoir tanks: Hold compressed or pressurized air to be used by the braking system
  • Drain valves: Release valves in the air tanks used to drain the air when the vehicle isn't in use
  • Foot valve (brake pedal): When depressed, air is released from the reservoir tanks
  • Brake chambers: Cylindrical container that houses a slack adjuster that moves a diaphragm or cam mechanism
  • Push rod: A steel rod similar to a piston that connects the brake chamber to the slack adjuster. When depressed, the brakes are released. If extended, the brakes are applied.
  • Slack adjusters: An arm connects the push rod to the brake s-cam to adjust the distance between the brake shoes
  • Brake S-cam: An s-shaped cam that pushes brake shoes apart and against the brake drum
  • Brake shoe: Steel mechanism with a lining that causes friction against the brake drum
  • Return spring: A stiff spring connected to each of the brake shoes that returns the shoes to the open position when not spread by the s-cam or diaphragm.

At idle (foot off the brake and vehicle's air system charged), air pressure overcomes the diaphragm or the s-cam is in the closed position, resulting in a released brake system. As soon as you depress the brake pedal, the air pressure decreases, turning the s-cam and spreading the brake shoes against the drum. The compressor refills the reservoir tanks and when you allow the pedal to retract, the air pressure increases back to the original state.

Emergency air brakes complement standard air-brake systems and can be activated by pulling a button on the dash (near the one with the light that we saw in the introduction). Before you can drive a vehicle with air brakes, you must push in the emergency brake button to fill the system with air. As long as the emergency system is pressurized, the emergency brake will remain free. If the system has a leak, the pressure can decrease enough to engage the emergency brake. In addition, heavy trucks are often equipped with an exhaust brake that aids the braking process, but this relies on the engine, not the air-brake system.

We've learned how air brakes work. Now let's look at how to maintenance can prevent brake failure in the next section.

Air Brakes: Preventative Maintenance

Poor maintenance of air brakes can lead to accidents.
Poor maintenance of air brakes can lead to accidents.
Andy Sacks/Getty Images

Every state in the U.S. has specific guidelines for operating a vehicle with air brakes. The tests to obtain a commercial driver’s license are demanding, as are the steps to maintain such a vehicle. Here are some steps you'll want to take before heading out on the road:

  • Make sure the minimum operating pressure for a vehicle air-brake systems is no less than 85 psi (pounds per square inch) for a bus and 100 psi for a truck.
  • 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. (This is called the air pressure buildup rate.)
  • Confirm that the correct cut-out governor pressure for the air compressor is between 120 psi and 135 psi. Cut-in pressure is 20 psi to 25 psi below cut-out pressure.

You'll also want to watch for water in the air-brake system, a byproduct of the condensed air. Air-brake lines don’t like water, especially in colder climates where ice can block air from reaching the brake mechanism and cause the wheel to lock up. To prevent this problem, many of the modern systems have automatic drain valves installed in each air tank.

Air couplers can also pose a problem. Worn rubber seals will cause air to escape. While the compressor can overcome a small leak, running compressors too hard can lead to failure. Again, as we’ve learned, air loss isn’t necessarily a bad thing, but it will mean you’re stuck. For truck drivers, getting stranded in the middle of a mountain pass probably isn't on the itinerary.

Brake sensitivity, another byproduct of air brakes, can lead to accidents, especially for inexperienced drivers. Air-brake systems are designed to work on vehicles carrying heavy loads. Have you ever wondered where all those dual skid marks on the expressway come from? That’s a product of light or empty trailers locking their rear wheels. Probably the worst fear for a truck driver is jackknifing. It’s never good when the back end of the trailer creeps up alongside the cab. Trucks traveling in rain and snow can easily jackknife if too much brake is applied.

Most modern vehicles with air brakes use a dual system. In essence, such equipped vehicles have two systems in case one should fail. Anti-lock brakes can now be found in tractor-trailer rigs and work much the same way as ABS systems found in passenger cars.

Fundamentally speaking, air brakes are efficient and reliable. However, don’t hold your breath if you’re hoping to find them in your car any time soon. Air-brakes systems occupy too much space and attention to be considered practical in cars. Just look at a Peterbilt truck as it saunters down the interstate. Have you seen the big tanks tucked behind the fuel tanks? Try finding a place for those under the hood of a Honda Civic.

If you’d like to learn more about air brakes and read some related HowStuffWorks articles, explore the links on the next page.

Air Brake Diagram

Air-brake components
Air-brake components

Now let's put the parts together to see how air brakes work as a whole. This diagram provides both a closeup view and an example of where the brakes are located in your vehicle.

Related Articles

More Great Links


  • Associated Press. "Another Deadly Parisian Train Crash." The New York Times. Aug. 7, 1988. (May 20, 2008)
  • California Department of Motor Vehicles. "California Commercial Driver Handbook Section 5: Air Brakes." Jan. 1, 2006. (May 21, 2008)
  • Carly, Larry. "Brake (device)." MSN Encarta. 2008. (May 17, 2008)
  • CDX Online eTextbook. "Braking Systems." (May 24, 2008)
  • CDX Online eTextbook. "Exhaust Brakes." (May 24, 2008)
  • Connor, Piers R. "Air Brakes." (May 18, 2008)
  • National Geographic Channel. "Seconds from Disaster; Runaway Train." (May 21, 2008)
  • National Transportation Safety Board. "Highway Accident Report PB97-916202." Oct. 17, 1997. (May 19, 2008)
  • San Diego Railway Museum. "Train Air Brake Description and History." (May 18, 2008)
  • Thomson, Clive. Canadian Underwriter. "Putting the Brakes on Air Brake failure." May 2007. (May 20, 2008)