Air brakes on trucks and heavy vehicles

Heavy vehicles weigh many times more than a car, and they must be able to brake safely with full load again and again without failure. Therefore trucks and buses typically use air brakes instead of hydraulic brakes. As a truck mechanic the brake system is one of the most important and most safety-critical areas you work with.

§Why compressed air?

Compressed air can be easily distributed throughout a large vehicle and then out to an attached trailer or articulated trailer. A compressor on the engine fills air tanks, and when the driver brakes, compressed air is sent to the brake cylinders at the wheels. The system can deliver the large forces needed to brake many tonnes, and it is robust against the repeated hard braking that heavy traffic requires.

§Spring brake — fail-safe by design

A key part of the system is the spring brake, which typically functions as parking and emergency brake. Here the brake is held open by air pressure, while a powerful spring will activate the brake if pressure is lost. This means that if a serious air failure occurs, the vehicle brakes itself instead of losing braking power. This is a deliberately fail-safe principle.

§Important parts of the system

• 01Compressor — fills the system with compressed air.
• 02Air bubbles, blisters or loose edges
• 03Air dryer — removes moisture so water does not accumulate in the system.
• 04Brake valves — meter air to the wheels according to pedal pressure.
• 05Brake cylinders and spring brakes — convert air pressure to braking force at the wheel.
• 06ABS/EBS control — prevents wheel locking and distributes braking force.

§Inspection and safety

When working on air brake systems you must depressurize the system correctly before separating parts and check for leaks, moisture and wear. Brake control on heavy vehicles is subject to strict requirements because the consequences of failure are great. Always follow the manufacturer's instructions and the applicable regulations for inspection and brake testing.