Usually when there's sand blowing back into the pool, it's because either it was just put through a backwash and not rinsed before returning to "filter" setting, or because there's crack in one or more of the laterals. It will happen sometimes too when the filter is too small for the pump. Since it's an older filter, I'm betting on the laterals. To check them, you need to take the top off the filter, remove the old sand until you get down to the laterals, which look like horizontal fingers about 1/2 way down. Usually the "fingers" will fold up like an umbrella so that you can lift the whole center assembly out. Then you need to carefully inspect each of them for cracks or breakage. It's not a hard job, really, just kind of labor-intensive. The last time we emptied ours, we used a shop-vac to suck it all out and it went much faster that way.

I'm not the equipment expert around here, and hopefully those that are will check in soon, but I would definitely start with the laterals and go from there.

Janet