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150 Year Old Technology Still Stops Big Vehicles – Here’s How Air Brakes Work

150 Year Old Technology Still Stops Big Vehicles – Here’s How Air Brakes Work

Larger vehicles like trains, buses, motorhomes, and tractor-trailers use air brakes instead of the hydraulic ones found in most standard cars. This is due to the heavier loads and increased cargo carrying capacities of these vehicles and the failsafe design of this system.

Hydraulic brakes rely on hydraulic fluid, whereas air brakes use air. Fluid can run out in the event of a leak, but air won’t. Let’s look more at how air brakes work and their advantages and disadvantages.

air brake pedal
Air systems require a longer stopping distance.

What Are Air Brakes?

In 1869, George Westinghouse invented the first triple-valve air-brake system for the new railroad emerging in America. Air pressure released the brakes rather than an operator manually using a hand brake. Westinghouse’s system is still in use today.

Large drum brakes
Unlike hydraulic brakes that use hydraulic fluid, air brakes use air to stop a vehicle.

In a hydraulic brake system, fluid pumps into pistons, pushing against the brake components. In an air system, an increase or decrease in air pressure dictates a proportional pressure change on the brake pads.

Heavier vehicles can’t risk having a leak and losing the hydraulic fluid. This is why trains, buses, motorhomes, and tractor-trailers will have an air brake system. However, standard vehicles don’t have them because of the size and complexity of the components involved.

Service vs. Spring Brakes

When driving a vehicle service brakes are used when your foot pushes the pedal. They may have air or hydraulic systems. When your foot presses the pedal a valve activates a system that allows air pressure (or hydraulic in a car) to pressurize lines that connect to the brakes. This pressure squeezes the brake pads against the rotor or drum to slow the vehicle.

Motorhome parking brake button
By pulling up on the parking/ emergency lever air is dumped from the spring brake lines. This engages the brakes and will stop the vehicle or hold it when parked.

All vehicles also have a second braking system. In a car this is known as a parking brake or emergency brake. In air brake vehicles this system is known as the spring brake system. Heavy-duty vehicles use spring brakes found on the rear axle. When the spring decompresses, the brake gets applied. Air pressure keeps the spring tightly coiled when driving. When air pressure drops, the spring uncoils and engages the parking brake, which is a safety feature in case of an air pressure leak.

So when you hear a bus or big vehicle puff a loud woosh of air when it comes to a stop, you are hearing the driver dumping the air from the spring brake system. By dumping the air the spring (or parking) brakes are applied and the vehicle will not move.

In the event of an air system failure, the spring brakes will apply and the vehicle will stop.

Air Brakes Explained Simply :: Service, Parking and Emergency Brakes One & the Same

What Vehicles Use Air Brakes? 

Heavy commercial vehicles, tractor-trailers, buses, trains, and some motorhomes all use air brakes. They can store the huge brake drums required for an air brake system and handle the additional weight of these drums and components. Plus, vehicles with multiple trailers need these systems for safer stopping power in the event of a leak.

Why Do Large Vehicles Use Them? 

When Westinghouse invented the air brake system first used in railway cars, he wanted to create a better, safer method of stopping. He didn’t want conductors going from car to car to apply the brakes manually. 

His invention involved a single pipe running underneath each railcar that refilled the reservoirs with air so that the conductor could pull one lever and the brakes throughout the train would engage. 

Westinghouse improved his invention, and in the early 20th Century, truck and heavy-duty vehicle manufacturers adopted his system.

air brake resevoirs
Compressed air stored in reservoirs helps stop heavy vehicles.

This system can’t run out of air like hydraulic ones can run out of liquid. Additionally, the compressed air stored in reservoirs has enough energy to stop a heavier vehicle if the compressor fails. These reservoirs can be located all along with the vehicle even as long as a train so each section can control its brakes. Large vehicle manufacturers choose to install these rather than hydraulic ones for these reasons.

Air Pressure gauges in  motorhome
Usually, vehicles will have multiple air systems, at minimum one for the front and one for the back of the vehicle. Its important to monitor these air tanks’ pressure.

➡ Your motorhome’s engine is just as important as its braking system. Find out Which Class A RV Type Is Better: Diesel Pusher vs. Gas Motorhome.

How Do Air Brakes Work?

There are two systems within an air brake system: supply and control. The supply system sends the high-pressure air to the control system. The air compressor draws in the air, compresses it, and stores it in reservoirs. The gauges in the vehicle’s cab help drivers watch these reservoirs.

The control system includes the service and parking brakes. When you engage the pedal, the air is routed from a reservoir to the brake chamber, thus slowing down the vehicle. The compressed air releases into the atmosphere once the driver disengages the pedal. 

The parking brake automatically engages when a sudden drop in air pressure occurs—the fail-safe system—or when the driver sets it manually.

What Are the Advantages? 

As already mentioned, air doesn’t run out like fluid. The energy stored in reservoirs can also stop heavy vehicles in case of brake failure. Additionally, you can install and remove air lines easier than hydraulic lines. 

motorhome chassis
The fail-safe system engages a spring to stop the vehicle if a drop in air pressure occurs.

Finally, in the event of a leak, it has a fail-safe system with a spring that stops the vehicle during reduced air pressure situations.

What Are the Disadvantages?

One of the biggest disadvantages is the cost. It’s more expensive to install them than hydraulic ones. Learning to operate them takes more time, and commercial drivers must undergo training.

Some states also require special endorsements for drivers of motorhomes that operate air brake systems. Owners of Class As must perform routine maintenance to ensure the system works properly. 

Additionally, sometimes you need air dryers to remove moisture buildup, leading to increased maintenance and repairs.

How Air Brakes Work On A Semi Truck

According to Good Sam, “The air pressure takes no more than 3 minutes to go from 50psi to 90psi at a near idle engine speed.” Additional checks include shutting off the engine and applying the brake to ensure that no more than a 3psi drop happens during one minute.

Finally, the release of compressed air is much louder than a hydraulic system. Some locations ban the use of air brakes because of this.

Are Air Brakes Hard to Use?

There are many inner workings within these systems, including chambers, valves, and reservoirs. Safely operating a vehicle with these brakes has a learning curve. In fact, commercial drivers must have an endorsement on their license, which means going through specialized training.

For both brake systems, you push down the pedal slowly until you come to a complete stop. In an emergency, you use controlled or stab braking if you have an anti-lock system installed. 

However, drivers need to know that air systems require a longer stopping distance. It has about a half-second delay for the air to flow through the lines to the brakes on larger vehicles with trailers.

Pro Tip: To avoid an RV accident, always leave enough space between your motorhome and the driver in front of you.

Finally, when parking a vehicle with an air brake system, you pull the handle to engage the parking brake. When you want to leave, you have to press the parking brake to disengage them.

Depending on how long you’re parked, you may need to wait a few minutes for the air pressure to build up again before releasing the parking brake. Your gauge should show the proper amount of air pressure.

air pressure gauges in motorhome
Its important to keep an eye on air pressures in a braking system.

Is Driving With Air Brakes Different? 

Air brakes do have a different pedal feel than hydraulic brakes as well. Many times the pedal will feel much stiffer and the brakes will feel grabby. This is just something the driver gets used to.

Additionally, owners must routinely inspect their systems to ensure safety. Brakes have slack adjusters that can be easily seen and need to be in proper adjustment. The air brake system will also need to be drained of air and checked for condensation once a month. Lastly a pressure test should be performed occasionally on the system which can be done from the driver’s seat.

If operating a vehicle with an air system was the same as driving a vehicle with a hydraulic one, the Federal Motor Carrier Safety Administration wouldn’t require special training. So before you jump into a vehicle with air brakes, make sure you know how to operate the system safely.

motorhome going down the highway

Are Air Brakes Better Than Hydraulic

In some ways, air brakes are better than hydraulic but they both have their place. On large vehicles, air systems are more reliable and work better over longer equipment. The complexity, weight, and operating characteristics make them less than ideal for smaller cars, however.

With newer electrically controlled hydraulic pumps, air systems may become less favored but for now, air brakes are still king of the road for big rigs.

Driving a large motorhome requires a different skill set than other vehicles, but is it more difficult? Find out: Is Driving a Class A Motorhome Hard?

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John

Tuesday 24th of May 2022

Describing the park brake in an air brake system you wrote: "When air pressure drops, the spring uncoils and engages the parking brake, which is a safety feature in case of an air pressure leak."

During normal operation of our big DP rigs all the braking is done by the same huge springs as the park brake system uses, correct?

In fact when we step on the air valve (brake pedal) we are actually releasing a bit of air pressure and the huge springs start the braking process. It's not air pressure pushing on the brake pads it the huge springs.

That's why air brakes are much better should there be a leak - brakes are automatically applied by the huge springs. If a hydraulic line leaks then all braking is lost.

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