Hi- I have a 45k ig pool with no main drain. Vinyl liner 9 years old, Hayward sand filter (big-no idea what size) and a 2.5hp 2 speed pump. I have been trying to vacuum for a few days-the vacuum will be fine for a few minutes and then the suction is so strong I need to turn off the pump to release the head. I have backwashed, rinsed, cleaned pump basket, even tried running it on recirculation- nothing keeps it from doing this. We replaced the sand this spring, and have only had to vacuum twice before- with no problems. I have maintained this pool for 20+ summers- never had this problem. Help!