Air brakes ARE drum-type brakes, but with an S-cam that actually moves the shoes outward to contact the drum surface. Likewise the hydraulic brakes, but they use hydraulic pressure to do the work.
Bear in mind that liquids are, by their very nature, incapable of being compressed to any measurable degree. Air brakes, OTOH, have about 150 lbs of air pressure behind them, much more than any hydraulic brake system.
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