Air Rams: How Pneumatic or Air Brakes Work

mechanic changes car brakes

Air brakes operate using air through pneumatic air rams. An air braking system maintains pressurised air inside its design, so the moment a driver applies the brake, pressure in the line starts dropping. The pressure drop results in the application of the brakes on the wheels. Air brake systems are used primarily on heavy vehicles such as trucks, buses, trailers, and trains.

Thank George Westinghouse for air brakes

Historically, before air brakes were invented, unfortunately, the heavy caboose for trains was first invented. Trains had to use a primitive brake system that required a brakeman in each car to apply a handbrake at the signal of the train engineer. This inefficient manual brake system caused an engineer named George Westinghouse in 1869 to invent a triple-valve air brake system to increase safety for the railroad industry. Though Westinghouse’s air brake system was a bit complicated, it did work well for trains.

Modern air brakes no longer use the triple-valve system, but still uses compressed air. When the brake is idle, air pressure overcomes the diaphragm, resulting in a released brake system. When the brake pedal is depressed, the air pressure decreases, spreading and turning the brake shoes against the drum. Then, the air pressure is applied to the brakes.

A basic air brake system contains five components: air reservoirs, air compressor, brake chambers, foot valves, and brake shoes and drums.

What is that Ppsssshhhh noise when the vehicle stops?

After a train, truck, or bus stops, the squeaking sound air brakes make is the air escaping after braking. That ppsssshhh sound is the automatic bypass safety valves. This ensures that the air pressure remains at the correct level. When the air compressor builds up too much air, the valves or air rams open, producing that loud hissing sound.

Why aren’t pneumatic systems used in light vehicles?

Though air brakes are better in braking efficiency and safer than hydraulic braking systems, pneumatic systems require a lot of space for a huge system with many components. A hydraulic system is small and light and doesn’t require much space for installation. Air brake systems are also expensive, which means that cars and other light vehicles would cost far more. 

Master Mac 2000

With air rams and other valve and cylinder systems, Master Mac 2000 has been the sole distributor of these components in Australia since 1989. Master Mac 2000 has grown to become one of Australia’s largest privately-owned exclusive distributors of quality brands such as Mack, Univer, Tolomatic, Piab, and American. We can service the country, particularly Queensland and Northern New South Wales, from our well-stocked office and warehouse in Brisbane. Call us today on 07 3344 4711 or visit our site at

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