Vehicle Inspections and Air Brake Checks for Combinations
Vehicle Inspections and Air Brake Checks for Combinations tests whether you can perform and interpret air-brake tests on a tractor-trailer, not just a single vehicle. Combination leakage limits are stricter than single-vehicle limits, and trailer reservoirs and the tractor protection valve change the test.
Air-brake tests
Build full pressure, then shut the engine off. With parking brakes released and the service brake applied and held, leakage should not exceed 4 psi per minute on a combination, compared with 3 psi on a single vehicle. With brakes released, the limit is 3 psi per minute on a combination, compared with 2 psi on a single. The low-pressure warning must activate by 60 psi, and the tractor protection valve should pop out between 20 and 45 psi. Confirm that trailer spring brakes set when the supply line is disconnected.
Pre-trip specifics
Check trailer reservoir drains, glad-hand seals, line chafing, slack adjusters, brake chambers, and pushrod travel. Confirm ABS lamps go out after the self-test. Verify trailer brakes hold with the trailer supply valve open and the tractor brakes released.
Worked example
A combination loses 5 psi in one minute with the service brake applied. The driver says the limit is 3 psi, so it passes. That is the single-vehicle limit; the combination limit is 4 psi, and 5 psi exceeds it. The unit should be placed out of service until the leak is repaired.
Traps
Testing with the parking brakes applied; using single-vehicle leak limits; skipping the trailer reservoir; ignoring slow compressor build-up; failing to check the low-pressure warning and tractor protection valve.