Ok so this is what I'd be looking for. Take the caliper off the mounting bracket and examine the slide pins. Look for scuffs/discolouration or obvious signs of wear. This will indicate if the slide pins or the bolts holding them in were loose or has been mechanically damaged/bent. Check the brake pad shims for wear, bending or gouging/scratches. Gently remove the rubber boots from the caliper, clean and flush them out thoroughly with degreaser and inspect for splits or deterioration. If you've made it this far and not seen any damage, buy a spray can of tire shine and a tube of marine grease. Hose the rubber boots down with tire shine and knead it into the rubber. This will rejuvenate the rubber and prolong their life (and makes them look like new). Install the boots into the brake caliper and apply the marine grease to reinstall the caliper. If your brake pad shims are gouged, replace them. Otherwise you can clean and polish them with fine grade steel wool and a metal polishing compound and paper towel. You can use fine grade steel wool and polishing compound to remove minor surface rust and crud from brake pistons as well but if they have pitting or gouges from being in contact with the caliper inner wall they need to be replaced.