Hydraulic Tank Cap

How often do you all clean or replace the cap on your hydraulic tank? I suppose they are all similar to the one on my New Holland. The design is an outer cap with a pleated paper element and a spring loaded pressure relief valve in the center. I wish I knew what pressure it is supposed to release at. I guess I could measure how many pounds of effort it takes to open the valve. It has a pretty stout spring in it and that gets compressed 70% or 80% when the cap is assembled. I can open it...

Hydraulic Tank Cap

Hydraulic Tank Cap