Welcome to the forum.
Is this an above or in-ground pool? Asking because we need to know if the water level is above or below the equipment. In either case I don't understand how air could be bubbling back into the pool when the pump shuts off. Since you do have leaks I can understand how water in the pump and filter would drain back to an in-ground pool....gravity. As for sand causing problens I doubt it unless you added something to the pool that's gunking up the sand OR something has been damaged in the filter. Sand doesn't go bad even after 15 years. Mine is pushing 14 or so. What kind of pressures are you seeing right after backwash and just before backwash ?
Al
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