[Beav]

The seal inside the caliper is 'square-cut', in other words its cross section isn't round like a normal 'o'-ring but square-faced. When hydraulic pressure is applied the piston is forced outward and the drag of the piston against this square-cut 'o'-ring causes it tip/slant/follow along (if the piston was traveling to the left, below the following lines, the 'o'-ring would move from this: |_| to this: /_/ ) When the hydraulic pressure is relieved the 'o'-ring relaxes to its natural state and retracts the piston to its original position. Now, you can imagine that there isn't much retraction force working here, so any build-up of gunk, etc. inside the caliper inhibits the amount of retraction. That's why rebuilding the calipers at each brake job, and/or flushing the brake fluid every couple years is recommended. (The gunk is comprised of moisture mixing with the brake fluid, deteriorating brake fluid and rubber parts and from sand casting residue still being released into the system from the master cylinder and calipers.)

That takes care of the inner pad on a single piston caliper. The outside pad retracts as a result of the caliper's mounting hardware or it just gets knocked back a bit by the rotor's deflection on its bearings during turns, etc.