The caliper pins are two round metal pins on each brake caliper where the brake piston assembly sits. They're called guide pins because they're responsible for guiding the proper angle for how the brake pad meets the rotor. If the caliper is not free to move, the culprit might be bent or binding mounting pins. Even if guide pins are not damaged, swollen or hardened guide seals or lack of lubrication may cause the caliper to bind resulting uneven pad wear. Always inspect all mounting surfaces and hardware for corrosion, wear or misalignment. Replace all mounting hardware and install a replacement caliper using the proper caliper lubricant.