Understanding How Air Brakes Work On A Commercial Motor Vehicle (CMV)

by Jordan Lebovitz | February 15th, 2017

Air brakes are likely the most important component part of modern commercial motor vehicles. The air brake system on a commercial motor vehicle (CMV), or semi-truck as commonly referred to, uses compressed air to push the brake shoe lining onto the brake drum creating friction to slow the wheel down. That may sound complicated – because it is. When slowing down a vehicle that can weigh 80,000 pounds, society is OK with complicated, so long as the air brakes work effectively, the driver knows how to use them, and the company who owns and services the trucks knows what to look for to adequately maintain the air brake system.

To ensure the air brakes are operating safely, you must first understand how the air brake operates. The following is a basic explanation how the air brake system works on a commercial motor vehicle.

The air is pumped through the air brake lines where it eventually reaches the air brake. Notice the gold rod on backside of brake which the air pressure pushes out.

The air is pumped through the air brake lines where it eventually reaches the air brake. The black brake shoe lining creates the friction and slow-down effect depending how much air is forced into the brake drum.

First, the air compressor pumps air into the air storage tanks, or reservoirs, that are stored beneath the truck. That air is then pumped through the air brake lines where it eventually reaches the air brake (pictured here). On most typical “S-Cam” brake systems, when you push the brake pedal on a commercial motor vehicle, air pressure pushes a rod out (see gold rod on back side of brake in photograph), moving the slack adjuster. The slack adjuster is used to essentially calibrate the brake system and ensure that the internal spring mechanism is working appropriately (aka the spring is not fully extended – which would not create the appropriate friction needed to slow the vehicle).

As shown in the photo below (taken at the Truck Driving Academy), air flows through the nozzle to the air brake chamber (the black/metal cylinder with gold rings around it), which then causes the spring to move the “S-cam.” The “S-cam” forces the brake shoe lining away from one another and presses them inside the brake drum. This creates the friction and pressure needed to slow the wheel down. Similarly, when you release your foot from depressing the brake pedal, the “S-cam” rotates back, causing the spring to pull those same brake shoes away from the brake drum eliminating the friction and slow-down effect.

The brake shoes, like a brake pad, wear over time especially with added pressure from depressing the brake pedal too often or too heavily. Safe practices require the commercial driver and company that operates a CMV to maintain their brake shoes at a 1/4 inch minimum thickness. Since both the brake shoe and the brake drum deteriorate over time and with excess heat/wear, it is essential that truck drivers and the companies they drive for are consistently and regularly checking the air brake system to ensure that there are no unsafe or malfunctioning components to the system.