I suspect you have a hole in the filter, or an o-ring or a gasket in the filter area that has failed. Take the whole filter assembly apart and you should be finding the problem. Since you may not see it with your eyes, perhaps you will need new parts, because the problem is only happening when the whole thing is assembled and the pump is running. God bless your efforts.
Answers & Comments
I suspect you have a hole in the filter, or an o-ring or a gasket in the filter area that has failed.
Take the whole filter assembly apart and you should be finding the problem.
Since you may not see it with your eyes, perhaps you will need new parts, because the problem is only happening when the whole thing is assembled and the pump is running.
God bless your efforts.