I have a Hayward sand filter (I'm not near it now but I believe it is a pro-series. In any case a standard sand filter with a control valve handle at the top to move between filter, waste, backwash, recirculate, etc.) It has been in use for seven seasons with no maintenance other than backwashing and rinsing. I know it's not working like it once was and I notice two things in particular. The pressure gauge rarely moves (it is working properly because I add DE occasionally and it moves then) and it simply does not clean the water as efficiently. What is this a likely sign of? Does it just need some maintenance or is the sand channeled? Does the sand need to be changed? Another problem is I see no way to remove the top without cutting the PVC pipes attached to the valve because they are firmly cemented together. Is that standard practice?