Air Brake System Components
How an air brake system actually moves a brake shoe — every part, in order.
Endorsement: Air Brakes · Source: FMCSA CDL Manual (public domain)
An air brake system has a fixed set of major components, and the exam expects you to name each one and explain its function. The compressor takes engine-driven mechanical power and pumps air into the system, controlled by the governor that cuts the compressor in and out at preset pressures (typically cut-out around 125 psi and cut-in around 100 psi). Air leaves the compressor and passes through an air dryer that removes moisture and oil before reaching the supply (or wet) tank.
From the supply tank, air feeds the primary and secondary service tanks (the wet/dry/dry configuration is the most common). The service tanks supply the foot valve, which the driver actuates with the brake pedal. Pressing the pedal sends air through brake lines to the brake chambers, large round metal cans mounted near each wheel. Inside each chamber is a flexible diaphragm that pushes a pushrod outward when air enters. The pushrod rotates the slack adjuster, which in turn rotates an S-cam that forces the brake shoes apart against the brake drum. Releasing the pedal lets air escape through quick-release valves, allowing return springs to retract the shoes.
Spring brakes are mounted in the rear of each chamber and serve as parking brakes and emergency brakes. They use powerful coil springs to apply the brakes mechanically; air pressure is required to hold them off. Loss of system pressure automatically applies the spring brakes, which is why spring brakes will set themselves if the system leaks down. The dual-air-system design means the front brakes and rear brakes draw from separate tanks, so a leak in one circuit still leaves the other functional. The low-pressure warning device (a buzzer or red light) must activate before pressure drops below 60 psi, giving you time to stop while you still have braking authority.
Key terms to memorize
- compressor
- governor
- supply tank
- service tank
- brake chamber
- slack adjuster
- low-pressure warning
- spring brake
- application gauge
Other Air Brakes topics
- Seven-Step Pre-Trip Air Brake Test — The mandatory pre-trip sequence that proves your brakes will work when you need them.
- Dual Air Systems — Why modern trucks have two independent air brake systems and how they protect you.
- Low-Pressure Warning Devices — How the system tells you that braking authority is about to be gone — and what to do.
- Parking Brake System — Spring brakes, holding power, and why you never use parking brakes as service brakes.
Test what you learned
Now that you have the Air Brake System Components material in your head, drill the Air Brakes practice test. The questions are drawn from the same FMCSA source material this article paraphrases. For state-specific framing, jump to your state page and pick the Air Brakes test for your jurisdiction.