How do air brakes work on a heavy vehicle?

Air brakes are the standard braking system on heavy trucks and buses in Quebec:

Operating principle:

• A compressor (driven by the engine) compresses air to 100-125 PSI (690-860 kPa)
• Compressed air is stored in reservoirs (usually 2 to 4 tanks of approximately 30 liters each)
• When the driver presses the brake pedal, the brake valve sends compressed air to brake chambers on each wheel
• Air pressure pushes a diaphragm that actuates the brake shoe against the drum (or the pad against the disc)

Feature: spring brakes:

• Parking brakes are spring brakes: a powerful spring applies the brakes when there is no air pressure
• If air pressure drops below 60 PSI, spring brakes apply automatically (safety in case of a leak)

Maintenance:

• Air tank drainage: daily (drain accumulated water and oil)
• Air dryer check: every 6 months
• Brake shoe adjustment: every 10,000 km or at each pad change