My Shimano EZ Fire shifters seem to have developed a small but irritating problem - in roughly midrange gears (on the rear) when I use the downshift lever it seems to get stuck. I can easily flick it back with my thumb with no effort, but it often catches me out.

My bike maintenance book says on later Shimano shifters there should be a screw which can be removed to allow lubricant to be squirted in but I can't find this. Has anyone found this screw?