Pneumatic Air Brakes

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Trucks and heavy motor vehicles carrying a heavy payload of 10 tons and above need tremendous stopping force. Hydraulics simply cannot deliver the torque necessary to bring such a heavy vehicle to an abrupt halt at short distances. But Pneumatic Air Brakes can and this is the reason they have been in widespread use in heavy trucks and heavy vehicles since the 1920s. One prime consideration is that air brakes are fail safe.


  • George Westinghouse is originally credited with having invented the air brake. He developed it for use in railway systems as far back as 1869 and refined the design leading to its adoption in heavy motor vehicles. His design included an air tank connected to a triple valve that kept brakes open through pressure springs. On release of pneumatic pressure the brakes clamped down on the wheel, making them more fail-safe than hydraulics.
  • Later on, Knorr-Bremse, a German company founded by Georg Knorr in 1905 developed pneumatic brakes with simultaneous four wheel braking facility for use in large trucks. This evolution lead to a widespread adoption of pneumatic brakes by the year 1925.

A further development was incorporation of antilock braking system during the 1960s.

Evolution and Adoption

  • Initial air brakes made use of a triple valve connected to an air tank through a pipe and a brake cylinder which applied the brakes on wheel once pressure reduced when air from cylinder was released.
  • Emergency brakes were an improvement, incorporating a second air tank and a quick action system, usually incorporated in trains to bring them to an immediate halt if the regular system fails.
  • Improvements on the basic design included incorporation of AB and ABD control valves. The AB system is the one currently in use in railway braking systems made up of two control valves and pipe bracket in addition to the brake. The ABD Valve system incorporated rubber diaphragms in place of brass pistons and shafts found in the older AB systems leading to faster response times.
  • Antilock Braking System (ABS) found its way into pneumatic air brakes starting back in 1920s but maturing in 1960s with Daimler Benz spearheading implementation of an electronic ABS four wheel air brake.
  • Electronic braking was pioneered by WABCO with a system that used electronics to control and monitor air pressure through the ECU now widely prevalent in modern trucks.

Within these categories there are different types of pneumatic air brakes such as:

  • Drum brakes similar to hydraulic and mechanical drum brakes.
  • Disc brakes making use of a rotor attached to the hub on which sits the caliper unit that squeezes the pads against the wheel.
  • Band brakes make use of a thick metal strip, curved or straight, with a friction pad
  • Cone brakes are shaped like a cone with wear resistant material to fit inside cup shaped surfaces.

The method of actuation may differ. Some pneumatic brakes are the spring return type while others are spring actuated.

Main components

Pneumatic or compressed air brakes have two main sections.

  • One is the supply system comprising of a compressor driven by the engine through a pulley. Air is transmitted to the storage tank through cooling and filtering units.
  • A four way valve connected to the tank distributes air to the front and rear brake air tank, parking brake tank and auxiliary air tank.
  • Supply side incorporates gauges and pressure limiting as well as safety valves.
  • The control system comprises of main brake, parking brake and trailer brake in case the truck has trailers.
  • The main brake circuit is divided into two parts, one for the front and second for the rear wheels.

Air Brake Activation is through the foot pedal. The parking brake may be hand operated.

How It Works

Operation of pneumatic brakes is simple.

  • Charging is the first process. The compressor pumps air into the system, storing it in the storage tank.
  • Applying brakes is the second process. Air at pressure of 100 psi and above is pushed to the brake chamber when the brake pedal is depressed causing the brakes to be applied.

The third stage is release in which increased pressure lead the triple valve to vent air to the atmosphere and recharge the storage cylinder with air.


  • Pneumatic air brakes can generate huge pressures and clamping force that will result in a heavily laden truck coming to a quick halt. Hydraulics simply cannot generate such pressure unless the system is cumbersome.
  • Pneumatic brakes are fail-safe. Air brakes rely on air pressure to force brake springs back. Should there be a failure in the air pressure system the brakes close and lock the wheels preventing the vehicle from moving.
  • Pneumatic systems are inherently lighter and require less maintenance besides being able to generate more force in comparison to a similarly sized hydraulic unit.
  • Air supply is limitless and there is no issue of failure as happens in the case of insufficient braking fluid in hydraulic systems.
  • Leaks in airline do not cause catastrophic failures.
  • Pneumatic circuit is easy to set up and maintain.
  • Multiple brakes can be coupled to a master pedal, as is found in trains.

In comparison to advantages, especially to stop heavy vehicles, disadvantages are few:

  • There is a slight time lag of about few milliseconds between actuating the pedal and application of braking force.
  • Temperature can make a difference to operation; especially freezing weather when there can be compression problems that cause delays.


Maintenance Tips

  • The pressure gauge must always be functional and the pressure should be maintained between 85 and 100 psi.
  • It is smart to check air pressure built up rate from 85 to 100 psi at engine RPM between 600 to 900.
  • Check that the cut out governor operates between 120 and 135 psi and cuts in at 20 psi below this pressure.
  • Check brake linings regularly and replace worn out pads.
  • Check S cam rollers and S cam bushings for corrosion and defective seals.
  • Check air tank and supply lines for leaks and leakage rate.
  • Check for brake imbalance—very important—by checking brake linings


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