The brakes used on large buses are different from those on regular passenger cars—they use an air brake system. The brake pedal on a vehicle equipped with air brakes looks similar to an accelerator pedal but is slightly larger. However, not all air brakes are the same. The brakes used on models like the Aero Town or Royal Midi are completely different in scale from those on Aero City-class and larger buses. Let’s take a closer look at air brakes, and in the process, learn a bit more about brakes in general.
Air Brakes
From Korean, AI-translated
Referring to the chart on the left, brakes are classified into parking brakes, service brakes, and retarder brakes. The parking brake is what we commonly call the handbrake or “side brake.” The service brake is the foot-operated brake used to stop the vehicle while driving. The retarder brake is an auxiliary brake that provides braking force via a separate device.
1. Parking Brake
The parking brake is used to park the vehicle or to hold it stationary on an incline. It is operated by a manual lever or auxiliary valve and is therefore also called the “hand brake.” It can also serve as an emergency brake if the service brake fails. Parking brakes can be either the Center Brake type or the Wheel Brake type. The Center Brake is not applied directly to the wheels; instead, it is mounted behind the transmission and applies braking force to the propeller shaft (P/Shaft) using the gear ratio of the rear axle. This design is mainly applied to the AOH type of medium-to-large hydraulic servo brakes.
Among vehicles with air brakes, older models such as the Aero Town, Royal Midi, early Royal City, and large trucks under 12 tons use the A.O.H type with the Center Brake method for parking brakes.
The Wheel Brake applies braking force mainly to the rear wheels. In passenger cars, the lever is attached to the service brake; in large commercial vehicles, a spring chamber is added to the service brake so that the parking and service brakes share the same mechanism. Vehicles with the spring brake type described later use this method for parking brakes. This method is used in newer Aero Town buses, all Hyundai large buses, Daewoo premium buses above model 116, and large trucks over 16 tons.
2. Service Brake
The service brake is used to stop a moving vehicle and is generally operated by foot—hence the name “foot brake.” The brake classification chart above shows that service brakes are divided into Hydraulic Servo Type and Air (Full Air) Type.
The Hydraulic Servo Type applies Pascal’s principle: the driver’s foot force is converted into hydraulic pressure and transmitted through pipes to the brake mechanism. Since foot force alone is insufficient for large braking power, engine-generated vacuum (max. 1 kgf/㎠) or compressed air (8–10 kgf/㎠) is used to boost the braking force. Depending on the boost method, it is divided into Vacuum Servo and Air Over Hydraulic (AOH). In the vacuum servo, pure hydraulic force operates the brakes—when the driver presses the pedal, vacuum pressure is added to the force, pushing brake fluid to actuate the brakes. The Air Over Hydraulic method (A.O.H) uses compressed air from an air tank to push brake fluid, activating the brakes.
Vacuum servo systems are common in passenger cars and light trucks up to 1 ton and are not considered air brakes. The A.O.H method is partially air-based and partially hydraulic. Vehicles with A.O.H brakes require an air compressor, an air tank, and brake fluid.
- Vacuum Servo Circuit Diagram 
- Air Over Hydraulic Circuit Diagram 
Full Air Type brakes use engine power to drive an air compressor, producing high-pressure compressed air (about 8–10 kgf/㎠) stored in an air tank. During braking, a control valve directs air into the brake chamber, pressing on the diaphragm to create braking force. The brake chamber controls both the foot brake and parking brake. It has multiple air ports, with a large one for the spring brake and another for the service brake. The spring brake uses the force of a powerful spring—when the parking brake is engaged, a loud hiss is heard as air is released, allowing the super-strong spring to expand and apply the brakes. Releasing the parking brake sends air into the spring line, compressing the spring and disengaging the brake. Pressing the foot brake sends air into the service brake line, pressing on the diaphragm to apply braking force. The spring inside is extremely strong—strong enough to cause fatal injury if the chamber is disassembled carelessly.
- Full Air Circuit Diagram 
The greatest advantage of the full air brake is that it does not use brake fluid at all. Since the brake chamber controls both parking and service brakes, there is no risk of brake failure from fluid leaks or vapor lock (when brake fluid boils, creating bubbles that prevent pressure buildup). Without brake fluid, vapor lock simply cannot occur.
In fluid brake systems, a leak or damaged pipe means complete brake loss. In a full air brake vehicle, if an air hose bursts and air escapes, the foot brake may fail—but the spring brake will automatically engage, functioning as the parking brake. This ensures that even at high speed, the vehicle can be brought to a safe stop, preventing serious accidents.
Another safety feature is that if the vehicle is parked and air pressure drops—whether due to a leak or repeated, heavy pedal use—the parking brake will not release even if the switch is returned to the off position until sufficient air pressure is restored. These are the reasons I emphasize the advantages of full air brake systems so strongly.
