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Have you tried adding 1/2 - 1 cup DE to the filter?

The DE will do one of 2 things

1) If you have broken internals, it will come right out of the retuns and let you know that it's time to replace parts or the entire filter.

2) If what's bypassing the filter is very fine debris, the DE will catch it and stop the problem.

A 5 lb box/ bag of DE is less expensive than a new filter and would, IMO, be the place to start.