Operation: High pressure brake fluid (red) forces the piston out and at the same time forces the whole caliper back to cause both pads to contact the rotor. The caliper "floats" on the caliper pin. When the brakes are releases, the hydraulic pressure drops. The low residual pressure (blue) allows the seal to draw the piston back thus freeing up the rotor and releasing the brakes. The seal twists and deforms when the brakes are applied and it is the recovery from this deformed state which causes the piston and pads to release.

Most systems allow a minimal pad drag even when the brakes are released however. Note that the piston can also slide through the seal providing a self adjusting action to compensate for pad wear.

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