Air brake systems are standard on heavy commercial vehicles. Instead of relying on hydraulic fluid pressure, they use compressed air stored in metal reservoirs. Compressed air is cheap, clean, and highly reliable. A minor leak in a hydraulic system will drain the fluid and kill the brakes completely, whereas an air system has a constant supply from the engine compressor, allowing the brakes to work even with minor leaks.
The system is split into two loops: the service brakes and the spring parking brakes. The spring brakes use heavy mechanical springs to lock the wheels when parked or if system pressure drops below 45 PSI, serving as an automatic emergency stop. Regular maintenance is vital, especially draining water from the storage tanks to prevent freezing in winter.