Driver Knowledge Tests

What are air brakes on a truck?

Air brakes on a truck are a type of braking system commonly used in heavy vehicles such as trucks, buses, and trailers. Unlike hydraulic brakes, which use fluid pressure to engage the brake pads against the rotors, air brakes use compressed air to perform the braking action.

How they work:

  1. Air compressor: An air compressor compresses air from the atmosphere, which is then stored in a reservoir tank. When the truck is first started, this air might need to be built up over a couple of minutes before it can be driven; if it takes longer than a few minutes when idling at 600-900rpm, there may be a leak. The overall pressure is controlled by the air compressor governor and is usually above 100 psi.
  2. Air brake system: The compressed air is distributed through a series of valves, hoses, and lines to the various brake chambers on the vehicle’s wheels.
  3. Brake chambers: Brake chambers convert the compressed air pressure into mechanical force to actuate the brake shoes or pads. When the brake pedal is pressed, air pressure is sent to the brake chambers, causing them to expand and push the brake shoes or pads against the brake drums or rotors, thereby slowing down or stopping the vehicle. Pushing the brake pedal will result in the noises of the bypass safety valves.
  4. Release mechanism: When the brake pedal is released, the air pressure is released from the brake chambers, allowing the brakes to disengage and the vehicle to move freely.
Air tank with the drain valve at the bottom. The driver pulls this to let any water escape that has not been stopped by the air dryer condensed in the tank. This water is very bad for the system, especially in colder climates where it can freeze and block the air lines.

Air brakes are commonly used in heavy vehicles because they provide reliable braking performance, especially under heavy loads or in demanding driving conditions. Air brakes have a built-in safety feature: if air pressure is lost, then the brakes are applied by a spring. This also means that if the air has been dumped (something that’s common to do with skeletal trailers when picking up a container, for example), air pressure will need to build back up again before the brakes can be released. However, proper maintenance and regular inspection of air brake systems are essential to ensure their safe and effective operation.

The air brakes are also called the service brakes. Truck drivers do not usually use the service brakes for slowing down, they use the engine brake, exhaust brake or retarder.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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