Air brakes

A loaded truck weighs many times what a car does, and a driver's leg cannot squeeze brake fluid hard enough to stop it. Air brakes solve this by using engine power to do the pushing. Every heavy truck and bus on Indian highways, from a Tata Prima or Ashok Leyland tractor to a BharatBenz tipper, uses them.

An engine-driven compressor pumps air through a dryer into storage tanks on the chassis. A gauge on the dash shows the tank pressure and a buzzer warns if it drops too low; a driver waits for the pressure to build before moving off.

• Pressing the pedal opens a foot valve that releases stored air in proportion to how hard you push.
• Air flows through pipes to a brake chamber at each wheel, where it pushes a rubber diaphragm.
• The diaphragm pushes a rod and a lever, the slack adjuster, which turns an S-shaped cam.
• The cam forces two shoes outward against the drum, exactly like a hydraulic drum brake but with air doing the work.
• Lift off and the chamber vents to the atmosphere: that loud hiss from a stopping bus.

Air brakes are fail-safe. A heavy spring inside each rear chamber is held off only by air pressure; if a pipe bursts or the tanks run down, the springs apply the brakes by themselves. The same spring is the parking brake, released by a knob on the dash. Trailers connect through the same system with a pair of coupling hoses. The downsides are a slight delay while air travels the length of the vehicle, a drum's tendency to fade on long descents (so trucks also use an exhaust brake) and more moving parts to keep sealed and adjusted.