I have a 25,000 gallon in ground pool and an American Products Titan CM 48 square-foot DE filter. A few years ago I replaced the spider gasket in my multiport valve.

Lately, I've noticed that 1-3% of the debris that I vacuum up gets blown back into the pool.

Where are the likeliest sources of the blow-by? Rip(s) in the filter grids (all but one is more than 10 years old)? There's a crack in the top (manifold) of the filter grid assembly but it's hairline and I have doubts about whether that could be the source. I haven't taken apart the multiport valve to check directly on the spider gasket.

Thoughts and information about previous experience appreciated.