Air-over-hydraulic brakes

Light and medium trucks are too heavy for pedal-and-booster hydraulic brakes alone but do not need the bulk and cost of a full air system. Air-over-hydraulic (AOH) brakes combine the two: air provides the muscle, brake fluid does the fine work at the wheels. Many LCVs and intermediate trucks in India, and the buses built on their chassis, use them.

The air side is a cut-down version of a heavy truck's: an engine-driven compressor, a tank and a treadle valve under the pedal. Instead of brake chambers at the wheels, the air goes to one or two air-hydraulic actuators, usually bolted to the chassis under the cab.

• Pressing the pedal opens the treadle valve and stored air flows to the actuator.
• Inside, air pushes a large piston, which is linked to a small piston in a hydraulic master cylinder.
• Because the air piston has a much larger area, the fluid comes out at a far higher pressure than the air went in.
• Ordinary brake pipes carry that fluid to wheel cylinders or calipers at each wheel.
• Releasing the pedal vents the air and the fluid returns.

For the driver the pedal is light and quick, more like a car's than a heavy truck's, with less delay than full air because the fluid lines are short. A separate handbrake, usually a lever acting on the transmission or on spring-brake chambers, parks the vehicle. The compromise is that there are two systems to maintain: an air circuit that must stay dry and leak-free, and a hydraulic circuit whose fluid must be changed. Loss of air leaves the driver with only what the pedal can push through the master cylinder by itself, which is very little on a loaded truck.