Pressing the brake pedal operates the brakes used for normal stopping. The brake pedal also controls the air pressure applied to the service brakes. As the pedal is pressed downward, compressed air passes through a valve attached to the brake pedal and is delivered to the brakes at the wheels. As the brake pedal is pressed harder, the valve opens further, delivering higher air pressure to the service brakes and increasing the braking force at the wheels.
Since most brake system designs use dual circuits, air is drawn from both the primary and secondary tanks and is directed to specific wheels on the vehicle. The wheels that receive air through the primary or secondary circuits vary, depending on the vehicle manufacturer. The dual circuit design means that if one circuit fails, the brakes will still operate on the wheels connected to the other circuit.