Pre-Trip Inspection of Air Brake Systems
Domain 3 covers the walk-around and in-cab checks that must be completed before driving. Georgia CDL pre-trip air brake questions expect a logical sequence: secure the vehicle, inspect components, drain moisture, build pressure, test for leaks, test warnings and spring brakes, and check adjustment.
Inspection sequence
- Chock the wheels. Check the compressor belt, air dryer, tanks, drain valves, hoses, lines, chambers, slack adjusters, pushrods, and drums or linings for cracks, chafing, leaks, or missing parts. Drain the tanks, especially the wet tank, daily.
- Start the engine and build to governor cut-out. Check that pressure builds from 85 to 100 psi within 45 seconds at operating RPM. Shut off the engine. Release the parking brake and hold the service brake for one minute; loss must not exceed 3 psi for a single vehicle or 4 psi for a combination.
- Pump down with the engine off. The low-pressure warning must activate before 60 psi; spring brakes must apply at 20-45 psi. Check pushrod travel and slack adjuster condition against the vehicle's adjustment limit. Check that the parking brake holds.
Worked example
After building to 125 psi and shutting down, a driver releases the parking brake, holds the service brake for one minute, and sees 6 psi loss on a single-unit truck. The limit is 3 psi, so the test fails. The driver should not sign off the inspection; the leak must be found and repaired. A common wrong conclusion is that 6 psi is acceptable because the gauge still shows pressure.
Traps that cost marks
Testing the leak rate with the parking brake applied can hide a service-brake leak. Do not skip the wet tank drain or test the warning with the engine running, because the compressor can keep pressure up. Do not accept a warning that first sounds at 55 psi; it must come on before 60. A visual check of slack adjusters does not replace measuring pushrod travel where required.
Key points
- Drain tanks, especially the wet tank, during pre-trip.
- Leak limit is 3 psi single and 4 psi combination.
- Build from 85 to 100 psi should take 45 seconds or less.
- Low-pressure warning must work before 60 psi.
- Spring brakes must apply between 20 and 45 psi.