I have a similar problem with my sand filter.
I have been battling algea and cloudy water for a month. I tried every chemical the store can sell then I noticed that my filters pressure had not gone up even though it had been running 24 hours a day for a week without back washing. I changed the sand then noticed that when I tried to vacuum the algea off the bottom it went right through the filter. I changed the spider gasket (same as pictured above) but same problem. After replacing the spider gasket there there is a lot of play in the valve handle. I pushed the gasket in as much as I could and there is no leaking from the backwash port. I used silicone spray to make it easier to get the gasket in place. The valve sits on top of a pvc return tube from the bottom of the tank. My next step will be to replace the O ring that goes around the tube. After that I an considering replacing the whole valve or maybe going to another type of filter although its only 4 years old.
It is a Columbia Valve and tank. Anyone have any experience with this companys products?