Inspecting the Air Brake System (Pre-Trip and In-Cab Checks)
This domain tests the pre-trip walk-around and in-cab air brake tests: what to look at, in what order, and which numbers mean pass or fail. Louisiana CDL exam questions often hinge on the leak-rate and build-up limits, so learn them precisely.
Walk-around
- Drain all reservoirs and watch for water or oil; a milky emulsion or oil means service is needed.
- Inspect hoses and lines for cracks, chafing, and leaks; check fittings.
- Look at slack adjusters and push rods: with the brakes applied, note the stroke and compare it to the manufacturer's limit; excessive travel means adjustment or repair.
- Check brake chambers for cracks, dents, and loose clamps; check linings and drums for cracks, oil, or thin friction material.
In-cab tests (engine off unless noted)
- Low-pressure warning: build to full pressure, shut off engine, fan the brakes. The light/buzzer must activate before pressure drops below 60 psi.
- Spring brake test: keep fanning; the parking brakes should apply in the 20–45 psi range.
- Static leak test: fully charged, engine off, parking brake released, no brakes applied - loss no more than 3 psi/min (single) or 4 psi/min (combination).
- Applied leak test: hold firm foot brake - no more than 4 psi/min single or 6 psi/min combination.
- Compressor build-up: with engine at fast idle, 85 to 100 psi in 45 seconds.
- Service brake test: at about 5 mph, stop; check for pull, delay, or unusual feel.
Worked example
A tractor-trailer is charged to 120 psi, engine off, brakes released. After one minute the gauge reads 115 psi. The drop is 5 psi, but the combination limit is 4 psi - fail, and the vehicle should be taken out of service pending repair.
Traps
Running the engine during a leak test, leaving the parking brake on, measuring build-up from zero instead of 85 psi, and forgetting that combination limits are higher than single-vehicle limits.