Inspecting and Using Air Brakes (Pre-trip and In-cab Checks)
Scope
Inspecting and Using Air Brakes (Pre-trip and In-cab Checks) tests the required sequence: walk-around inspection, compressor build-up, governor check, leak tests, low-pressure warning, spring brake check, and slack adjuster inspection. The exam wants the order and the pass/fail limits.
Core rules
- Check slack adjusters by pulling them by hand with brakes released; free play should be about 1 inch or less, and pushrod travel must be within the limit for the chamber type.
- Build air pressure at fast idle. A common CDL limit is 85 to 100 psi within 45 seconds.
- Note governor cut-out and cut-in, usually near 120–125 psi and 100 psi.
- Static leak test: engine off, fully charged, parking brake released, wait one minute. Common limits are 3 psi for a combination and 2 psi for a single vehicle.
- Applied leak test: hold the brake pedal; after the initial drop, loss should not exceed 4 psi per minute for a combination or 3 psi for a single.
- The low-pressure warning must come on before pressure falls below 60 psi. The parking brake valve should pop out at about 20–45 psi.
Worked example
From 85 psi, the gauge takes 70 seconds to reach 100 psi. The build-up test fails because the limit is 45 seconds. Possible causes include a worn compressor, a loose belt, a governor problem, or a leak. Recheck the gauge and belt, but do not pass the vehicle until build-up meets the limit.
Traps
Do not leave the parking brake applied during the static leak test. Do not skip the applied leak test. Do not accept a warning that comes on only after pressure is already below 60 psi.