Air brake system.
Air brake system. A system, including an air-over-hydraulic brake subsystem, that uses air as a medium for transmitting pressure or force from the driver control to the service brake, but does not include a system that uses compressed air or vacuum only to assist the driver in applying muscular force to hydraulic or mechanical components.
49 CFR § 393.5
As used in this part, the following words and terms are construed to mean: