Janet's post in another thread details how to service the filter.
It is titled... for those with sand blowing back..... you may have already found it.
We had problems with sand flowing in pool this year too. We replaced the multiport valve, gasket and checked the laterals after emptying sand using wet dry vac and replaced sand. (We decided to replace valve because it was cracking from exposure to elements)
It wasn't too big of a job, but then again, I was just the gopher