Skip to content

How Do Air Brakes Work?

A set of air brakes under a vehicle.

Chances are good that you don’t regularly drive a vehicle with air brakes, but you’ve probably ridden in one. For example, trucks, buses, trailers and semi-trailers use air brakes, which provide several advantages to these larger vehicles. But how do air brakes work? Let’s take a look.

Hydraulic Brake Systems

Hydraulic brakes are found on passenger cars with both disc and caliper brakes. They use incompressible brake fluid to transfer pressure from the brake pad and shoe to the rotors and drums until the wheels stop. This system works well for most passenger cars because it doesn’t take much fluid to operate, and the brake system is designed to take up minimal space.

Air Brake SystemsHow Do Air Brakes Work?

Air brakes use compressed air instead of hydraulic fluid and were originally designed for trains. They have an unlimited supply of air and can still operate during a minor leak. On the road, air brakes are used on larger trucks and heavy commercial vehicles. These systems use an air compressor driven by the engine to build up and store air in tanks at a predetermined and regulated pressure. 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 slow or stop the vehicle.

Air pressure actively holds back large springs that enable the emergency brake. If the system loses pressure due to an air leak or disconnect, the brakes will automatically engage. As you step on the brake pedal, pressure in the system is released, deploying the brakes gradually. This requires the compressor to build up more pressure, so the brakes are held in the open position. Air brake systems come equipped with alerts to let you know if you’re losing too much pressure.

Emergency Brake Systems

The built-in nature of the emergency brakes and the limited supply of air make air brakes extremely reliable, but there are drawbacks. Air brakes take up a lot of room and require careful and frequent maintenance to remain reliable. They also have a tendency to develop condensation, which can be a hazard in freezing climates, and the braking experience is anything but smooth.

All in all, they’re a rather ingenious system and are the best choice for larger modes of ground transportation. Technological advancements will likely improve these systems with time, but it’s unlikely that you’ll see them on passenger vehicles anytime soon.

Check out all the air brake products available on NAPA Online, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how air brakes work, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photos courtesy of Blair Lampe.


Blair Lampe View All

Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter.  In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.

Leave a Reply

Your email address will not be published. Required fields are marked *