Air Brake Inspection and Pre-Trip Checks
Air Brake Inspection and Pre-Trip Checks
This domain tests the sequence and pass/fail limits for the air-brake pre-trip inspection: build-up, static leak, warning-device, and spring-brake checks. The exam expects numbers and correct test conditions, not a vague "check the brakes."
Core rules
- Build-up test: run the engine at fast idle. In a dual system, pressure should rise from 85 to 100 psi within 45 seconds. Slower build points to compressor or governor trouble.
- Static leak test: charge the system fully, shut the engine off, release the parking brake, and apply and hold the service brake. After the initial gauge drop, loss must not exceed 3 psi in one minute for a single vehicle or 4 psi for a combination.
- Low-pressure warning check: with the engine off, fan off pressure. The warning must come on before 60 psi.
- Spring-brake check: keep reducing pressure. Parking/emergency spring brakes must apply in the 20-45 psi range.
- Slack adjusters and pushrods: with brakes released, mark the pushrod, apply the brake, and measure travel. Excessive travel (over about 1 inch at the slack adjuster) means adjustment is needed; automatic slack adjusters should not be manually readjusted as a substitute for repair.
- Also inspect chamber hoses, drums, linings, and air lines for cracks, chafing, and leaks.
Worked example
On a tractor-trailer, the gauge drops 2 psi in the first minute after the initial drop, then holds at 1 psi for the next minute. The test passes because the limit for a combination is 4 psi per minute after the initial drop. If the same unit lost 5 psi in that minute, it would fail the leak test and should not be dispatched.
Traps
Running the engine during the static leak test invalidates it. Leaving the parking brake applied can also mislead you. Do not count the initial drop as part of the one-minute leak limit. Do not use the single-vehicle 3 psi limit for a combination. Do not skip the warning/spring-brake sequence because the leak test passed.