Air-over-hydraulic brake systems operate using a combination of compressed air and hydraulic pressure, and the release of the air and hydraulic pressure engages the master cylinder. It then applies pressure to the valves, and the brake fluid is released. The brakes then exert friction on the tires, which stops them.
Air-over-hydraulic brake systems contain an air-over-hydraulic power cylinder. This cylinder has both an air cylinder and a hydraulic cylinder inside, which work in tandem with each other. The air and hydraulic cylinders each contain a piston and common rod. The air cylinder's piston is a larger diameter than the hydraulic cylinder's piston. This results in greater hydraulic pressure being released during normal breaking. When the brake pedal is pressed, the valves are opened by the master cylinder, releasing the atmospheric pressure that builds up when the valves are closed. During hard braking, the master cylinder causes greater valve movement. This results in the valve releasing more air into the air-over-hydraulic power cylinder, allowing for faster braking.
Most modern vehicles have two-circuit brake systems, which are operated by the master cylinder. Each circuit controls two wheels. This increases safety because if one circuit fails, drivers can still brake as long as the other circuit remains operational.