Air Brake System Inspection and Testing
What the exam tests
Inspection and testing questions ask for the Ontario pre-trip air-brake test sequence and the pass/fail limits. You must know what to check, in what order, and what result makes the vehicle unsafe.
Test sequence and limits
- Build pressure to governor cut-out, then shut the engine off. With brakes released, leakage should generally not exceed 2 psi per minute on a single vehicle; with brakes applied, not more than 3 psi. Combination units have slightly higher allowances.
- The low-pressure warning must operate before pressure falls below 60 psi. Spring brakes should begin to apply in the 20–45 psi range.
- Check compressor build-up: at fast idle, pressure should rise from about 85 to 100 psi within a reasonable time, commonly 45 seconds.
- Measure pushrod travel at full brake application. A Type 30 chamber commonly has a 2.5-inch limit; Type 20 is about 2 inches. Know the chamber type and limit for your vehicle.
- Check slack adjuster angle, chamber condition, hoses, glad hands, drain cocks and air dryer operation.
Worked example
You apply the brakes fully and measure a Type 30 chamber’s pushrod at 2.75 inches. The limit is 2.5 inches, so the brake is out of adjustment. Do not simply shorten the pushrod on an automatic slack adjuster and pass it. Diagnose why the adjuster is not maintaining stroke-worn S-cam bushings, a leaking chamber, a damaged slack adjuster or incorrect installation-and repair the cause.
Traps that cost marks
Measuring pushrod travel with the brakes released; skipping the low-pressure warning check; accepting a slow pressure build-up; and treating a leak test as passed without letting pressure stabilize. Do not confuse the warning-pressure threshold with the spring-brake application range.