Air Brakes
What this domain tests
The Air Brakes domain tests how a CMV air system builds, stores, and uses pressure, how to test it, and what happens when pressure falls. Expect questions on warning devices, spring brakes, leak tests, and safe braking technique.
Core rules
- The compressor builds pressure; the governor cuts out near 120–135 psi and cuts in near 85–105 psi.
- A low-pressure warning must activate by 60 psi. Spring brakes begin to apply at roughly 20–45 psi, so stop before pressure falls that far.
- With the engine off and full pressure, static leakage should be no more than about 3 psi per minute on a single vehicle and 4 psi per minute on a combination.
- Drain air tanks daily to remove moisture and oil; in cold weather, moisture can freeze and disable the system.
- Brake with a steady, controlled application. Do not fan the brakes; repeated applications can exhaust pressure and overheat drums.
Worked example
A driver shuts the engine off at 125 psi and watches the gauge for one minute. It drops 5 psi. For a combination vehicle, that exceeds the 4 psi-per-minute limit, so the driver should not dispatch the vehicle. The reasoning: leakage that fast can outpace compressor output during repeated braking, leaving too little pressure to release spring brakes or stop safely.
Traps
Using the trailer hand valve as a parking brake; fanning brakes down a grade; ignoring a compressor that cycles too often; and assuming spring brakes are only for parking. They are emergency brakes too.