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Air brakes take longer to stop than hydraulic brakes primarily because it takes longer for the air to flow through the lines. In an air brake system, the braking action is initiated by compressing air, which needs to travel through hoses and chambers to create the necessary force to push the brake shoes against the brake drums or discs.

The process involves several steps that require the air to build up pressure and then move through various components like valves and brake chambers, which can introduce delays compared to hydraulic systems where brake fluid is instantly transmitted throughout the lines. Hydraulic systems rely on incompressible fluid, allowing for a more immediate response when the brake pedal is pressed, leading to faster stopping characteristics.

While the other options mention factors like efficiency of air pressure, density of hydraulic fluid, and the complexity of the system, they do not directly address the flow dynamics that contribute to the time it takes for brakes to engage, which is why the flow of air itself is the key factor in understanding the longer stopping distance associated with air brake systems.