The callipers "slide" on two steel guide pins. There are rubber boots on the callipers that the guide pins fit into. These are supposed to be fitted dry (no grease) and to work correctly (allowing the calliper to centre correctly on the brake rotor) the guide pins need to be clean and smooth, with no brake dust build up or pitting etc. The rubber boots also need to be in good condition too, and not missing the plastic caps on the end of the boot.
These pins and boot are often over-looked in a rebuild.
Parts 4-6 in this image...
