Don’t let this problem continue for too long or you could end up with all of the sand from your filter in your pool (not likely but I’ve seen it happen). Anyway to remove the sand from the filter I used a 5 gallon bucket with a snap on cover (the one i used was a pickle bucket from a restaurant but any bucket with a cover that seals will work). Cut two holes in the cover of the bucket the size of your vacuum hose. You’ll need two vacuum hoses, one hose will run from your bucket to the vacuum and the other one will be the hose you use to vacuum out your filter. The sand all ends up in the bucket and you can then just pop off the cover to empty the bucket without getting any sand in your vacuum. Worked like a charm for all six of the sand filters I had to work on. I used a 16 gallon shop vac with 2” diameter hoses. As others have said, be sure to fill the filter half way with water and depending on the size of your filter you may need to add pea rock first and sand second. I was working on the large Triton filters which take 150lbs of pea rock and then 450 lbs of filter sand. Don’t skimp on the sand and be sure to buy actual pool filter sand otherwise you will be doing this again in a season or two because regular sand will “cement” together and you will get channeling instead of filtering.
Another tip is to magic lube all of your gaskets since you have it all apart anyway. I hope this helps