Air Brakes
The Core Idea
Air brake questions test the components, the warning signs, and the test sequence. Know the parts and what each does, and know the pressures at which warnings trigger.
Concepts You Must Know
- Components: compressor, governor, air tanks (wet and dry), safety valve, service brake chamber, spring (parking) brake chamber, slack adjusters, and the low-pressure warning device.
- Governor cut-in/cut-out: the compressor builds air to about 125 psi (cut-out) and resumes at about 100 psi (cut-in).
- Low-pressure warning must activate before pressure drops below 60 psi; the spring brakes should apply when pressure falls to roughly 20-45 psi.
- Air brake tests: check for leaks with the engine off, do a static leak test (loss should be no more than 3 psi in one minute for a single vehicle, 4 psi for a combination), and a compressor build-up test (pressure should rise from 85 to 100 psi within 45 seconds).
- Slack adjusters: pushrod travel beyond the limit means brakes are out of adjustment; you cannot adjust them if the pushrod is over-stroked.
Traps to Watch
Questions often swap the 60 psi warning threshold with the 20-45 psi spring brake application. Another trap is confusing a leak with a build-up problem. Remember: with the engine off and brakes released, a steady pressure drop signals a leak.
Rule of Thumb
"60 warns, 45 to 20 parks." The low-pressure buzzer/light must come on before 60 psi, and the spring brakes fully apply somewhere between 20 and 45 psi.