Most large commercial vehicles use air brakes, which are vastly different from the hydraulic brakes in your car. Understanding how they work and how to properly inspect and use them is absolutely essential for every CDL driver. Without air, your brakes won't work, and that's a situation no one wants to be in!

How Air Brakes Work

Instead of fluid pressure, air brakes use compressed air to create the force needed to stop the vehicle. Here's a simplified breakdown:

1. Air Compressor: This is the heart of the system. It builds up and maintains air pressure in the air tanks. It's driven by the engine.
2. Air Storage Tanks (Reservoirs): These tanks hold compressed air. There are typically three tanks: a primary, a secondary, and a parking brake tank.
3. Foot Valve (Brake Pedal): When you press the brake pedal, it opens a valve that allows compressed air to flow from the tanks to the brake chambers.
4. Brake Chambers: These are located at each wheel. When air enters a brake chamber, it pushes on a diaphragm, which moves a pushrod.
5. Slack Adjusters: The pushrod is connected to a slack adjuster, which in turn rotates the S-cam.
6. S-Cam: The S-cam (shaped like an "S") rotates and forces the brake shoes away from each other and against the inside of the brake drum.
7. Brake Drums and Shoes: Friction between the brake shoes and the rotating brake drum slows and stops the wheel.
8. Spring Brakes (Parking/Emergency Brakes): These are held off by air pressure. If air pressure drops too low, or you pull the parking brake knob, springs engage the brakes, bringing the vehicle to a stop. This is your emergency braking system!

Key Components and Gauges

You'll see a few important things related to air brakes in your cab:

• Air Pressure Gauges: Usually two of them, showing the pressure in the primary and secondary air tanks. You need between 100-125 psi.
• Low Air Pressure Warning: A light and/or buzzer that activates when air pressure drops below 55-60 psi (or about half the cut-out pressure on older vehicles). This is a serious warning!
• Safety Valve: Located on the air tank, it's designed to release air if pressure exceeds 150 psi, preventing tank rupture.
• Drain Valves: Located at the bottom of air tanks to drain water and oil that condense in the system. Failing to drain them can lead to brake failure!

Air Brake Inspection Procedures

A critical part of your pre-trip inspection involves testing the air brake system:

• Check Air Compressor Governor Cut-in and Cut-out Pressures: The compressor should start building air at around 85 psi and stop ("cut-out") at 125 psi.
• Test Low Air Pressure Warning Signal: Turn off the engine with the key on, step on and off the brake pedal to reduce air pressure. The warning should activate before 60 psi.
• Check Spring Brakes (Pop-Out Test): Continue reducing air pressure. The parking brake knob should "pop out" (spring brakes apply) when air pressure drops to between 20-45 psi.
• Static Air Leakage Test: With the engine off, air pressure built up, and brakes released, note the initial pressure. After one minute, pressure drop should be no more than 2 psi for a single vehicle or 3 psi for a combination vehicle.
• Applied Air Leakage Test: With the engine off and brakes applied, note the initial pressure. After one minute, pressure drop should be no more than 3 psi for a single vehicle or 4 psi for a combination vehicle.
• Check Slack Adjusters: Visually inspect and pull on the slack adjusters. They should not move more than 1 inch when pulled manually.