What does the term 'dual air brake system' refer to?

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1. A system with two separate compressors that always run at the same time, when a dual system actually means two separate circuits fed from a single compressor, not two compressors
2. A term for a system that mixes air pressure and hydraulic fluid in the same lines
3. A system with two separate air circuits, usually one for the front axle and one for the rear, so a leak in one does not disable both
4. A system found only on buses, never on straight trucks or tractor-trailers

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