Air Brakes
How air brakes work and the required system checks.
4.1 Parts of an air brake system
- Air compressor pumps air into the tanks; the governor controls when it pumps (cut-out is usually around 125 psi and cut-in around 100 psi).
- Air storage tanks must be drained to remove water and oil (manual or automatic drains).
- Safety relief valve usually opens at 150 psi.
- Low air pressure warning must come on before pressure in the service tanks falls below 60 psi.
- Spring brakes are held back by air; they come on automatically when air pressure drops (usually between 20 and 45 psi).
Note: Brake lag — air brakes take a little time to apply after you press the pedal. This adds about 32 feet to stopping distance at 55 mph.
Source: AAMVA / FMCSA — Commercial Driver License Manual (AAMVA model manual used by state CDL manuals) (Model manual — verify your state edition). Updated: 2026-10-09
4.2 Air brake system checks
| Check | Typical standard (dual air system) |
|---|---|
| Air pressure build-up | 85 to 100 psi within 45 seconds |
| Leakage, brakes released | Single vehicle ≤ 2 psi/min, combination ≤ 3 psi/min |
| Leakage, brakes applied | Single vehicle ≤ 3 psi/min, combination ≤ 4 psi/min |
| Low air warning | On before pressure falls below 60 psi |
| Spring brakes | Set automatically, usually 20–45 psi |
Check At what pressure must the low air warning activate?
Source: AAMVA / FMCSA — Commercial Driver License Manual (AAMVA model manual used by state CDL manuals) (Model manual — verify your state edition). Updated: 2026-10-09
4.3 Braking on long downgrades
Use a low gear and let engine braking do most of the work. When you reach your "safe" speed, brake hard enough to feel a definite slowdown. When speed is about 5 mph below your safe speed, release the brakes. Repeat as speed rises again (snub braking).
Warning: Fanning the brakes or riding them on a long grade causes fade and can lead to total brake failure.
Source: AAMVA / FMCSA — Commercial Driver License Manual (AAMVA model manual used by state CDL manuals) (Model manual — verify your state edition). Updated: 2026-10-09