In my years (55+) of working on bikes I've run across this a couple of times. Usually high mileage bikes that never had the appropriate lube applied to the slider pin/bolts, front and rear. I worked on a pretty high mileage one a year or two ago and not only was the slider bushing rusted and worn but the pad retainer pins were almost worn through! The front still had some of the original machining marks on the discs! I inquired as to why the parts had never been lubed. The owner said that it was stupid to put any kind of lubricant anywhere near the brakes. As far as the pad pins, He said he NEVER uses the front brake, other than holding it in place on an up hill stop sign. Everyone knows that using the front brake will cause you to flip! Who knew?