Have a bad check valve as found the little flapper laying in the bottom of the filter .Ordered a replacement and it came so tried to install 'Mine says push tab back and pull straight up so I assume no snap ring and no glue . Well after twisting and pulling for 45 minutes today it would not budge . Gives you a funny feeling as you pull and tug that your going to break the filter housing . Any tricks to get this out and the new in?

I thought would run without but think the issue is that my DE at pump shutdown flowed back into pool through the skimmer - am I correct you must have this check valve or leave pump run 24 hrs? Any help on tricks to get this out would be appreciated