How Do Air Brakes Work - Ultimate Truckers Guide

How Do Air Brakes Work – Ultimate Truckers Guide

Source: Youtube

Further more, the use of an Air brake system allows for the combination of an emergency breaking system that puts in use parts of the service brake as well as the parking brake systems.

The Air brake systems have three braking systems combined:

  • The service brake system;
  • The parking brake system; and
  • The emergency brake system.

Most of the class 8 trucks have Air brakes; they have 4 wheels drum Air brakes.  Now, of course there are some trucks that have 4 wheels disc hydraulic brakes, but those types of trucks will take bigger effort and distance to stop than the vehicle that has Air brake system.

Source: Youtube

I believe that the main reason why the Air brake equipped vehicles stop better, is because they have drum brakes, which have more surface area than disc brakes. And we all know that more friction area equals better brakes!

So basically Air brakes and their advantages only come into play on heavy vehicles. At least that is what I believe, and most people will agree with me I am sure!

Why do Cars Only Have Hydraulic Brakes?

It is hard to tell, but wouldn’t they stop quicker with Air brakes?

Well, yes, you are probably right for thinking this. Especially if you take in consideration the hydraulic fluid in the car braking systems, which can run out in the event of a leak!


And here we go again, disc vs. drum, the ongoing debate since the don of time!

Most cars have 4 wheel disc brakes despite being problematic and prone to warping. Since modern disc brakes can lock up all wheels (without using ABS), there is no chance to make them stop better.

They’ve reached maximum performance in regards to stopping distance, so isn’t Air brakes the next logical improvement?

I guess not yet, at least not for now!


Benefits to Air Brakes

  • No fluid to house or cool, so the brake maintenance is more simple;
  • If your Air brakes get too hot they will “fail” and your vehicle will not stop;
  • Even though Air brakes are not fade proof, they can evaporate heat a lot better than the hydraulic brakes;
  • You can use drums and still have better stopping distance than disc brakes without the warping pulsating discs.

Air Brakes Disadvantages

  • Air compressor (bigger load on engine) bigger possibility to freeze up on you when you take in consideration winter time and humidity, while Ail brakes won’t freeze;
  • The poor pedal feel, but a lot of hydraulic brakes have poor pedal feel too.

Rules and Requirements for Air brakes

If we put in consideration that all heavy haul transportation is weighed down by heavy passenger or cargo loads, we will agree that safety is of the utmost importance.

Just as I mentioned safety, all those government agencies run through my head!

As we all know emergency braking systems are required on all semi-trailers by the CFR 49 393.43.

I wish that was the only law, and that we can stop right here, but trucking industry is so diversified, that pretty much every government agency has a say, with their own rules and regulation for obeying.


Here, I will mention just few of the most common agencies that you will need to pay attention to, and constantly read their websites for updates:

The Air Brake System! What Is It Made Of? How Do Air Brakes Work?

The Air brake system is using air for transmitting pressure from the drivers control to the service brake. Moreover, it includes an air-over-hydraulic brake system also.


The Air brake system contains:

  • Air Dryer;
  • Alcohol evaporator;
  • Safety valve;
  • Brake pedal;
  • Air compressor and governor;
  • Drum brakes;
  • S-cam brakes;
  • CamLaster;
  • Wedge brakes;
  • Disc brakes;
  • One-way check valve;
  • Air supply pressure gauge;
  • Application pressure gauge;
  • Low Air pressure warning gauge;
  • Stop light switch;
  • Front brake limiting valve;
  • Tractor protection valve;
  • Trailer air supply control;
  • Trailer air lines;
  • Service air line;
  • Emergency air line;
  • Spring brakes;
  • Service, spring, and emergency brakes;
  • Parking brake controls;
  • Anti-compound system;
  • Modulating control valves;
  • Dual parking control valve;
  • Dual Air brake systems;
  • Combination vehicle Air brakes;
  • Hose couplers;
  • Trailer Air tanks;
  • Shut-off valves;
Source: Youtube

I wish I could say some more about each component that the Air brakes have, but I guess that’s topic for one of my next articles, since I need to focus here on explaining how do Air brakes work?

Why Do Trucks Use Air Brakes? How do Air Brakes Work With Trucks?


There are millions of reasons why the heavy trucks are using Air brakes,  but as the most obvious I will give these two:

  • Air brakes are much more practical (especially when you are  repeatedly coupling  and uncoupling the truck from the trailer).
  • No need to bleed brake fluid of air (the fluid is air).

Sure, Air brakes  have their shortcomings too, such as the more aggressive application and the initial expense,  but they are definitely way better system for heavy set vehicles.

When a hydraulic system rests, there is no pressure in it. That is how they are set up; there is no braking power, until you apply pressure.

On the other side, with the Air brake system, there is air that is  under pressure. That pressure is around 125 p.s.i. and is  in the tractor and the trailer.

With this pressure you can apply brakes available only a few feet away from the brakes. When the air supply is close to the wheels, next thing you need to apply the brakes is a “signal” pressure from the brake pedal.



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