Ability to connect a trailer or multiple trailers quickly. When you remove air for the system (like leaving a trailer somewhere) the brakes are locked. If you lose air pressure they fail with the brakes applied. If there’s a small leak the trucks air compressor can compensate until it’s fixed. Repairs are much faster and can be performed roadside. You can hear air leaks. Air lines don’t rust.
Many other systems are controlled by air. The truck and trailer suspension, automatic transmission, cab suspension, axle lockers, radiator fan. All controlled with air pressure.
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