Air Brakes
Domain 3: Air Brakes
The Air Brakes domain tests system knowledge and the tests you must perform: compressor and governor operation, dual air reservoirs, warning devices, spring brakes, slack adjusters, and the static, applied-leakage, low-pressure-warning, and spring-brake checks.
Core rules
- The compressor should build pressure from 85 to 100 psi within 45 seconds at 600–900 rpm.
- The low-pressure warning must activate before pressure falls below 60 psi. Spring brakes apply automatically at about 20–45 psi.
- With the engine off and brakes released, pressure loss should not exceed 3 psi in one minute for a single vehicle; with brakes applied, not more than 4 psi per minute.
- Drain air tanks daily to remove moisture; in cold weather, drain more often.
- Do not fan the brakes on a long downgrade. Use a lower gear and apply steady, light braking.
Worked example
During the low-pressure warning test, build system pressure to governor cut-out, shut off the engine, and pump the brake pedal. The warning buzzer or light must come on before the gauge drops below 60 psi, and the spring brakes should set at 20–45 psi. If the warning fails, the vehicle is out of service - the test is not a formality.
Traps that cost marks
Confusing the warning threshold with the spring-brake application range; skipping tank drainage; failing to check slack adjuster travel; and releasing the parking brake before pressure is sufficient.