Air Brakes on CMVs – How they work & regulations you need to know

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In 2018, the FMCSA handed out 185,592 violations to drivers for not having properly adjusted brakes (393.47E Brakes (All Others) – Clamp or Roto Type Brake Out of Adjustment). Not only is this an extremely dangerous problem to have, but it’s also avoidable with a little background on the different components that make up an air braking system.

Calibrating an air braking system is pretty straightforward. Watch this video to see the process:

Brief Explanation of how Air Braking Systems Work: 

An air compressor mounted to the engine block fills the air tanks up to around 90-105psi. Air is routed to the push/pull valves in the cab which are used to apply or release the brakes. When the valves are pulled out, the air is cut off and the brakes automatically push against the drums (given the proper brake shoe adjustment). When the valves are pressed in, the air flows through the brake lines to the brake chambers which releases the outward spring pressure. The Brake Chamber push-rod attaches to a Slack Adjuster and the slack adjuster turns an S-Cam that applies pressure directly to the brake shoes.

Here’s what the air system looks like on the power unit:

The FMCSA takes brake safety very seriously. With all the coverage on the Colorado incident, this is sure to become an even more common topic of conversation online and during roadside inspections. Whether we like it or not, drivers and companies are going to have to educate themselves on proper brake inspections as well as procedures in the event of brake overheating or air-system failure. Otherwise, we will see more drivers and/or companies being held liable.

Section 393.47 of the FMCSA rule books says:

(a) General requirements. Brake components must be constructed, installed and maintained to prevent excessive fading and grabbing. The means of attachment and physical characteristics must provide for safe and reliable stopping of the commercial motor vehicle. 

(b) Brake chambers. The service brake chambers and spring brake chambers on each end of an axle must be the same size. 

(c) Slack adjusters. The effective length of the slack adjuster on each end of an axle must be the same. 

(d) Linings and pads. The thickness of the brake linings or pads shall meet the applicable requirements of this paragraph – 

(1) Steering axle brakes. The brake lining/pad thickness on the steering axle of a truck, truck-tractor or bus shall not be less than 4.8 mm (3⁄16 inch) at the shoe center for a shoe with a continuous strip of lining; less than 6.4 mm (1⁄4 inch) at the shoe center for a shoe with two pads; or worn to the wear indicator if the lining is so marked, for air drum brakes.

The steering axle brake lining/pad thickness shall not be less than 3.2 mm (1⁄8 inch) for air disc brakes, or 1.6 mm (1⁄16 inch) or less for hydraulic disc, drum and electric brakes. 

(2) Non-steering axle brakes. An air braked commercial motor vehicle shall not be operated with brake lining/pad thickness less than 6.4 mm (1⁄4 inch) or to the wear indicator if the lining is so marked (measured at the shoe center for drum brakes); or less than 3.2 mm (1⁄8 inch) for disc brakes. Hydraulic or electric braked commercial motor vehicles shall not be operated with a lining/pad thickness less than 1.6 mm (1⁄16 inch) (measured at the shoe center) for disc or drum brakes.

For more information on brake adjustment specs, see

A little knowledge can go a long way in keeping your trucks on the road. Making sure you and your drivers are familiar with the braking systems and adjustment requirements is a small step you can take to avoid downtime and fines.


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FMCSA Resources

What is the Federal Motor Carrier Safety Administration

The FMCSA is the lead federal government agency responsible for regulating and providing safety oversight of commercial motor vehicles (CMVs), to include more than 500,000 commercial