A hydraulic line with fittings is the pressure-carrying connection in a hydraulic brake system.
What it does (simple + technical)
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It carries hydraulic brake fluid from the master cylinder (lever or pedal) to the brake caliper
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When you pull the brake lever, pressure is created in the master cylinder
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That pressure travels through the hydraulic line
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The pressure pushes the caliper piston, which squeezes the brake pads against the disc