Core Components

An air brake system is split into three primary, interconnected circuits:

• Supply System: Features the engine air compressor, air dryer (to remove moisture), and storage reservoirs.
• Service Brake System: Applies and releases the brakes during normal driving via the foot pedal.
• Parking/Emergency Brake System: Utilizes heavy springs that apply the brakes automatically if the system's air pressure drops below a safe threshold (typically 60 PSI).

Types of Wheel Brakes

At each wheel end, the compressed air pushes into a brake chamber to force friction materials against the spinning wheel assembly:

• S-Cam Drum Brakes: The most traditional heavy-duty design. The air chamber pushes a pushrod and slack adjuster, which rotates a shaft with an S-shaped end. This "S-cam" pushes the brake shoes outward against the inside of a rotating brake drum.
• Air Disc Brakes: Increasingly common on modern trucks. Similar to passenger cars, the air pressure squeezes a brake caliper, forcing brake pads against a flat metal rotor. They offer shorter stopping distances and better fade resistance.