It is common in the agricultural industry to operate equipment, like semis and ten-wheelers, to carry tons of weight in feed, forages or even to haul manure. It is critical to know how to be able to stop, and stop evenly, while maintaining safe control of the vehicle. In this article, I would like to focus on a subject that plays a key role in the operation of heavy equipment – air brake systems.

Schlund jim
Retired Diesel Mechanics Professor / College of Southern Idaho

When the early braking systems came out, they did a really good job of slowing down and stopping the vehicle. However, back then they didn’t have jake brakes. Emergency brake systems are now also part of air brake systems.

As we’ve evolved through the braking systems, we’ve seen it go from just a means of stopping the piece of equipment to being able to better control the vehicle in a safe manner while maintaining the ability to steer it.

Today, anti-lock braking systems (ABS) are installed on most if not all new vehicles. This system pulsates the brakes, keeping your wheels from locking up, which will avoid skids and slides by sensing tire rotations.

When it doesn’t see that rotation, it automatically pulses the relay valve off and on, creating a steady stop. Keep in mind though that there are still vehicles out there that don’t have this system.


Air brakes
Air brakes became popular because of their tremendous stopping power and long life. Basically, brakes are the parts of the vehicle that absorb the energy generated by a vehicle in motion in order to stop it.

In order to do this, a system with several components is needed. Air provides a simple outlet to accomplish this task.

Air brakes are able to do this based on the way they are designed. Air brakes require the use of three braking systems: service brakes, parking brakes and emergency brakes.

Air brake system key players
A big advantage of air brakes is that they have a tremendous ability to be able to apply a lot of pressure to the linings on the drums in order to stop, and they have a big surface area to absorb heat generated from the braking.

There are several key players in the system including a compressor, check valves, a dryer filter, reservoir (supply) tanks, control valves, the foot treadle, emergency brake and parking brakes.

In order for the system to work efficiently there has to be a way to pump, store and replenish air. That’s what the compressor does. It compresses air, sending it to reservoir tanks while keeping a certain pressure level.

The compressor runs all the time and most are operated using pistons. The compressor is mechanically driven by the engine, is cooled by the engine’s cooling system and lubricated by the engine’s oil.

All air that leaves the compressor also goes through a dryer filter, which removes the impurities from the air.

The air compressor governor controls when the compressor will pump air into the supply tanks by keeping track of when the air tanks’ pressures rise or fall below a certain level.

A series of check valves are located throughout the braking system. These valves are designed to keep air in the supply tanks, which will be used for the service brakes, emergency brakes and trailer brakes.

Service, emergency and parking brakes
Service brakes are the brakes that are used during normal driving and are controlled by the foot treadle. When you step on the brake treadle while you are going down the road, you’re signaling to put the brakes on.

Stepping on the treadle to stop the vehicle causes air to go down the service or control line from the supply tank to the brake chambers. At the chamber, the air pressure causes the brake shoes to press against a break drum (in most cases it is an S-cam drum).

Letting up on the treadle will reduce the air pressure from the supply tanks which, in turn, releases the brakes.

This creates a balanced system and an even stop. This is similar to how the brakes in your car work. If you push down on the brake pedal easy, it slows you down easy. If you push down hard, it stops harder.

The emergency brake system uses parts of the service and parking brakes systems. If working properly, this system will take over and stop the vehicle in case of a brake system failure.

When it comes to parking, the maxi-brakes or spring brakes are applied. Maxi-brakes house internal mechanical springs that are activated by the air pressure loss. The air pressure holds these brakes open in normal driving conditions.

When the air pressure can no longer overcome the pressure exerted by the springs, these brakes activate. These brakes are primarily used to safely secure the vehicle when parked.

Trailers now have maxi-brakes as well that do the same thing, but it is still advisable to use wheel chocks as an additional safety measure.

This is only a brief explanation of the air brake system. Our goal is to help you have a basic understanding of the air brake system.

In a future article, we’ll discuss problems that you may encounter with your air brake systems, how to troubleshoot them and avoid them. PD

See the second part of the series on air brake systems "Mechanics Corner: Problems with air brake systems."


Jim Schlund
Retired Diesel Mechanics Professor
College of Southern Idaho