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 fairly easy with a couple of small screwdrivers. I am tempted to drill and tap the tank and put a zero to 25lb gauge on it to monitor how much pressure it gets up to. Not sure what if any damage it would do when clogged. I know a clogged vent on an axle or transmission will cause seals to start leaking. I have run into that on several vehicles and my little John Deere hydrostatic tractor. Mine seemed to have a lot of pressure built up when I removed it today so I cleaned it for the second time in a year and a half. I pry the outer case off and wash it in diesel fuel to start. Then I wash it out with carb or brake cleaner and blow it dry with compressed air. Here is what it looks like disassembled.
It is supposed to hold pressure in the tank, I forget how much but I think it's around 5-7 psi. If you are working at a slant this will keep oil flowing to your pump suction.