Air Brake System Fundamentals
Air Brake System Fundamentals
This domain tests how compressed air is produced, stored, controlled and turned into braking force. Saskatchewan air-brake questions often ask you to explain why dual circuits, pressure protection and spring brakes exist-not simply to name parts.
Core rules
- An engine-driven compressor supplies air through an air dryer to reservoirs. The governor unloads the compressor at cut-out pressure and restarts it at cut-in, usually about 20 psi lower. Many heavy vehicles cut out near 120–125 psi; confirm the vehicle specification.
- Reservoirs include a wet/supply tank and primary/secondary circuits. A dual-circuit system is designed so a failure in one circuit still leaves braking in the other.
- The foot valve sends a control signal; relay valves deliver large air volumes to brake chambers, reducing lag.
- Service brakes are graduated by air pressure. Parking and emergency braking use spring force: air pressure holds the spring compressed, and losing air allows the spring to apply the brake.
- Foundation brakes convert chamber force into friction. Pushrod travel, slack-adjuster geometry and lining condition determine usable braking force.
Worked example
A tractor has governor cut-out at 125 psi and cut-in at 105 psi. During a stationary leak check the combination loses 4 psi/min with the service brake applied. The common combination limit is 4 psi/min, so this is at the limit, not a comfortable pass. Reasoning: the 20 psi governor band is normal; the problem is leakage rate. Recheck fittings, hoses and valves, and repair before dispatch.
Traps
Do not reverse cut-in and cut-out. Do not say spring brakes are applied by air-air releases them. Do not assume fast buildup proves there are no leaks; the compressor can mask leakage. Do not claim one circuit failure causes total loss in a dual system. Do not ignore the wet tank: water and oil carried forward damage valves.