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The piston on a disc caliper never returns to the bottom of the caliper's cylinder; it stays out, almost in contact with the rotor at all times. This feature is the reason discs are called "self adjusting". So as the pads wear, additional fluid must be available to fill the space behind the pistons.
With drum brakes, the pistons retract to the bottom of the cylinders after each application of the brakes. The volume never changes. The brake shoes require constant mechanical adjustment to stay in proximity of the drums. In the olden days, this required backing up and hitting the brakes, or crawling under the car with a large screw driver.
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