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Pistons that transfer hydraulic force to the pads.
Pitted or seized pistons can cause uneven pad wear or dragging brakes. Match pistons by diameter and material. Clean the caliper bore and use new seals during fitting. A smooth piston action keeps braking even across both sides.
Moisture in old brake fluid and damaged dust boots allow rust to form.
Yes. Worn seals can leak or allow dirt to enter.
Light marks can be cleaned, but deep pitting means replacement.