I recently required a bike that is probably about ten years old with Shimano RSX 8-speed components. The bike obviously was barely ridden and everything's in great shape, but both shifters are having issues. They will both shift up, but not down. As in the larger brake lever shifts, but the smaller lever doesn't shift or even click or move the cable at all, although I can push the lever in. The levers were doing this when I got them, and they felt gunky probably from old caked-up grease, so I've been flushing with wd-40 and tri-flow. This has made them feel better shifting up, but still no action shifting the other way. It almost seems like they've been somehow set into one speed but I'm not sure if that's possible.
My next step will be taking the levers apart as I've heard it's doable with these old 8 speeds. However if anyone has any input into this particular problem it would be greatly appreciated.