Air Brakes on a Passenger Vehicle: Pre-Trip Tests, Pressure Limits, and Safe Use
What this domain tests
Air Brakes applies when the passenger vehicle is equipped with air brakes. The Delaware CDL exam tests pre-trip air-brake inspection, pressure tests, warning devices, spring brakes, and safe brake use on grades. You must know the test sequence and the failure limits, not just the names of parts.
Core rules
- Pre-trip checks: Drain moisture from air tanks. Check the compressor build-up rate, governor cut-out, low-pressure warning, spring-brake application, leakage, slack adjusters, brake chambers, and parking brake.
- Pressure standards: The compressor should build from 85 to 100 psi within 45 seconds. The low-pressure warning must activate before pressure falls below 60 psi. Spring brakes must apply between 20 and 45 psi.
- Leak test: With the engine off, parking brake released, and service brake applied, pressure should not drop more than 3 psi in one minute for a single-unit passenger bus (4 psi for a combination vehicle, if applicable).
- Safe use: Use controlled braking, do not fan the brakes, select a lower gear before a downgrade, allow for air-brake lag, and set the parking brake and chock when parked.
Worked example
On pre-trip, the engine is running and pressure rises from 85 to 100 psi in 70 seconds. That exceeds the 45-second build-up standard, so the bus fails the air-brake test. Separately, with the engine off and service brake applied, pressure falls 6 psi in one minute; that exceeds the 3 psi/min limit for a single unit. Either defect keeps the bus out of service until repaired.
Traps that cost marks
Memorizing one pressure for every check; skipping tank drainage; ignoring slow build-up; not testing the low-pressure warning; confusing 3 psi and 4 psi leak limits; fanning brakes downhill; forgetting the parking brake or chocks; assuming air brakes react instantly.