Air Brakes
What this domain tests
Air brake questions test component knowledge, build-up and leak tests, low-pressure warning, spring brake operation, and stopping technique. You must know what the gauges and tests are telling you.
Core rules
- The compressor builds pressure; the governor controls cut-in and cut-out, typically near 100 psi and 120–125 psi.
- A typical build-up check: pressure should rise from 85 to 100 psi within 45 seconds.
- With brakes released, leakage should not exceed about 2 psi per minute for a single vehicle or 3 psi for a combination. With brakes applied, the limits are about 3 and 4 psi.
- The low-air warning must operate before pressure falls to 60 psi. Spring brakes apply at roughly 20–45 psi.
- Do not fan the brakes on a long downgrade. Select a safe gear before the grade and use controlled braking.
- Check slack adjusters and pushrod travel; a brake out of adjustment may not apply fully.
Worked example
After the system is fully charged, the driver shuts off the engine and releases the parking brake. The gauge drops 2.5 psi in one minute in a single vehicle. The limit is about 2 psi, so the leak is excessive and the vehicle should be placed out of service until repaired. A 1.5 psi drop in the same test would be within the typical limit. The reasoning is the rate, not the total drop over several minutes.
Traps that cost marks
Confusing brake lag with bad brakes; skipping the leak test; ignoring a low-air warning; fanning brakes downhill; and assuming a slack adjuster is correct because the pushrod looks straight.