If when you replace the Turbine paper filter elements and the filter is clean near the top and dirty below, this indicates that fuel is not being drawn through all of the filter. This may indicate a vacuum leak if the filter was full to the top when closed up. This is a potential safety hazard and could get you killed if the fuel level were to fall below the feed holes in the filter housing and shut the engine down near land, rocks or other obstacles, or while crossing a bad river entrance.
If the check valves are not closing properly in the filter housing, the fuel level may fall during the short period while you are tightening up the filter, cap or in the case of a spin-on filter the filter body. This air can eventually lead you to grief. I have had it happen. This business of knowing if the check valves are working is not simple, I think it will require more explanation that I can presently give here.
It is my experience that a good number of check valves over 10 years old are no longer working. In the case of spin-on filters with priming pumps built-in that the pump will NOT prime. In that case you can test rather easily, no priming action: bad check valve.
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