Think the air brake system on heavy duty trucks is confusing? Think again. The mechanics are quite straightforward and function similarly to the brake systems on almost all other trucks.

Think the air brake system on heavy duty trucks is confusing? Think again. The mechanics are quite straightforward and function similarly to the brake systems on almost all other trucks.

At its most basic level, all air brake systems use air pressure, which is stored in a series of tanks, to apply the brakes when a driver steps on the pedal. The air itself is pressurized by the compressor on the truck’s engine and is regulated by the air governor. Most systems, if not all, work on 120 psi of pressure.

Going further, some systems have an air dryer, which takes the moisture out of the air to keep it from freezing in the wintertime - very important for our temperamental North American climate. (It's also important to note  here that the air dryer has a cartridge in it that needs be changed once a year).

Once the air has passed through the tanks, it goes into two different systems:
Emergency System - This system releases the parking brake and holds some reserve air to stop the truck if there is a leak. The parking brake system functions by activating springs in the air chambers on the rear axles of the truck. When the parking brake is activated, air is supplied to the chambers and pushes against the springs to release the brakes.

For safety reasons, if the pressure in the air system pulls below 60 psi, the parking brake knob will pop out and set the brakes. For those driving semi tractors with an attached trailer, the knob for the brakes functions in the same way. Additionally, if there is a leak in the trailer, the tractor protection valve will set the brakes to protect the tractor's air supply, ensuring the vehicle can still stop safely.

Service Brake System - Simply put, this system stops the truck when the brake pedal is depressed. When pushed, the air goes through to the pedal valve which modulates the air flow to the brake chambers. The brake chambers, in turn, push on the slack adjusters, which help keep the brakes in adjustment. The slack adjusters turn a shaft that turns the S cams, which push the brake shoes out into the drums. The S cam shafts have bushings in them that need to be checked and greased every time you change the shoes.

Trucks can use two types of slack adjusters. Manual, which must be manually adjusted them to keep the brakes running properly, and automatic, which automatically adjust themselves. Many drivers recommend manual adjusters, as automatic adjusters need be greased (in particular, white lithium grease) in order to prevent them from seizing.

Finally, there are also air disc brakes, although they are not very common. Most brake shoes have an indicator built into the end of the pads that advise when they need to be replaced. When replacing brake shoes, invest in hardware kits that include the springs, pins and bushings for the brakes. When replacing brake shoes, it's a good idea to replace the drums at the same time, as many will have wear ridges and heat cracks.

While there are a number of different moving parts in play, the function of an air brake system is quite simple. As long each of the parts are maintained, serviced and replaced in a timely manner, you will be able to rely on your air brakes every time you press down that pedal.