The mechanics are straightforward and function similarly to the brake systems on almost all other trucks.

All air brake systems use air pressure stored in a series of tanks to apply the brakes when a driver steps on the pedal. The air is pressurized by the compressor on the truck's engine and 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 essential for our temperamental Canadian climate. (It's also important to note that the air dryer has a cartridge that needs to 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 activates springs in the air chambers on the truck's rear axle. When the parking brake is activated, the air is supplied to the rooms and pushed 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-tractor trucks with an attached trailer, the knob functions similarly. 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 - This system stops the truck when the brake pedal is depressed. When pushed, the air goes through to the pedal valve, which modulates the airflow to the brake chambers. The brake chambers, in turn, try 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 camshafts have bushings that must be checked and greased every time you change the shoes.

Trucks can use two types of slack adjusters:

Manual, which must be manually adjusted to keep the brakes running correctly, and; Automatic, which automatically adjusts itself. 

Many drivers recommend manual adjusters, as automatic adjusters need to be greased 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 brake springs, pins, and bushings. It's also a good idea to replace the drums simultaneously, as many will have wear ridges and heat cracks.

While there are several different moving parts in play, the function of an air brake system is quite simple. As long as each piece is maintained, serviced, and replaced promptly, you can rely on your air brakes every time you press the pedal.