Air Brakes
Air Brakes tests whether you understand the air-brake system, can perform its required tests, and know how to use it without overheating or losing control. Expect questions on warning devices, pressure buildup, leakage, slack adjusters, spring brakes, and brake fade. This domain is about system behavior, not just pedal use.
Core rules
- The compressor, governed by a cut-in/cut-out cycle, charges reservoirs. Typical cut-out is about 120–125 psi and cut-in about 100 psi.
- A low-pressure warning light and buzzer must activate before pressure drops below 60 psi. If the warning fails, the vehicle is not safe to drive.
- With a dual-circuit system, pressure should build from 85 to 100 psi within 45 seconds at fast idle.
- Leak test with engine off, parking brake released, service brake applied and held: loss should not exceed 3 psi per minute for a single vehicle or 4 psi for a combination.
- Spring brakes apply automatically when pressure falls to roughly 20–45 psi. They are parking/emergency brakes, not a substitute for the service brake.
- Check slack adjuster free play and pushrod travel. Excessive travel means brakes need adjustment or repair.
- Control speed downhill with a low gear and snub braking; do not fan or ride the brakes.
Worked example
A combination vehicle is fully charged at 120 psi. The engine is shut off, the parking brake is released, and the service brake is held. After one minute the gauge reads 118 psi. The loss is 2 psi, below the 4 psi combination limit, so the leak test passes. If the same vehicle lost 6 psi, it would fail and must not be dispatched until repaired.
Traps
- Fanning the brakes on a long downgrade instead of using a low gear.
- Ignoring a warning that comes on below 60 psi.
- Using spring brakes as service brakes.
- Forgetting to drain air tanks and check for moisture.