Hi i have a five year old sand filter witch is working great and my water is sparkling clean.Should i open it up and clean of the dirt on the top or should i just leave it the way it is till i start having problems.
Not at all! If you're going to backwash, make it worthwhile.
What I'm saying is that gaskets and O-rings get hard and brittle with time, and take a set. Disturbing them may cause them to start leaking. I don't know what style backflush valve you have, but if it's one of those old up-n-down push-pull valves, you may all the sudden see water dribbling out around the shaft. You may see strange things happen with pressure, you may see fine sand spewing out your returns. Or maybe nothing will happen. Just for amusement, go use the search box at the bottom of the page and enter "spider gasket," then read some of the posts. Then you won't be blindsided.