Inspecting Air Brake Systems
The Inspecting Air Brake Systems domain tests the pre-trip and in-cab checks that prove the system builds, holds, and warns correctly. The exam expects a sequence, not a random walk around the truck.
Core checks
- Drain tanks: open drain cocks with the system charged and look for moisture and oil. Close them before testing.
- Build-up and governor: start the engine and watch pressure rise. A common standard is 85 to 100 psi within 45 seconds at fast idle. Confirm cut-out and cut-in.
- Static leak test: charge the system, shut the engine off, release the parking brake, and watch for one minute. A common limit is less than 3 psi loss for a single vehicle and less than 4 psi for a combination.
- Applied leak test: hold a full service brake application for one minute. A common limit is less than 4 psi loss.
- Low-pressure warning: pump down with the engine off. The warning must activate before pressure drops below 60 psi.
- Spring brake test: continue pumping down; spring brakes should apply in the 20–45 psi range.
- Visual checks: brake chambers, hoses, slack adjusters, pushrod travel, drums, linings, and mounting hardware.
Worked example: On a combination vehicle, you charge to 100 psi, shut the engine off, release the parking brake, and wait one minute. The gauge drops 5 psi. That exceeds the common 4 psi limit, so the vehicle fails the static leak test until the leak is found.
Traps: checking leaks with the engine running; confusing static and applied tests; skipping the low-pressure warning; measuring pushrod travel without comparing it to the manufacturer’s limit.