Commercial Vehicle Pre-Trip Inspection and Air Brake Checks
What this domain tests
The pre-trip inspection and air-brake portion of the Ontario Class A exam checks whether you can find defects before they become roadside failures and whether you can perform and interpret the air-brake tests in the correct sequence. You are expected to know acceptable pressure loss, warning thresholds, and when a defect makes the vehicle unfit to drive.
Walk-around essentials
- Complete the daily inspection before first use and every 24 hours; record defects and do not drive with a major defect.
- Check tires and wheels for cuts, exposed cord, missing nuts, cracked rims, and leaks; check lights, mirrors, fluids, load security, coupling, and emergency equipment.
- Confirm the parking brake is set and wheels are chocked before crawling under or adjusting anything.
Air-brake test sequence
- Build pressure to governor cut-out, usually about 120–125 psi (830–860 kPa), then shut the engine off and release the parking brake.
- Static leak test: pressure loss should not exceed 2 psi/min on a single unit or 3 psi/min on a combination.
- Applied-brake leak test: hold the brake pedal; loss should not exceed 3 psi/min single or 4 psi/min combination.
- The low-pressure warning must activate before pressure falls below 60 psi (414 kPa); spring brakes should apply in the 20–45 psi range.
Worked example: A tractor-trailer loses 8 psi over 3 minutes with brakes released. The limit is 3 psi/min, so the allowable loss is 9 psi; 8 psi is within the limit, but a 10-psi loss would fail and require repair before dispatch.
Traps: forgetting to release the parking brake during the leak test, testing with the engine running, mixing psi and kPa, and ignoring an inoperative warning buzzer because the brakes still stop the vehicle.