Yours is a priority valve that has a diverter to enable changing flow. It is for your power steering. It makes sure there is always flow for the power steering. The diverter part only changes what is left after power steering to the port that feeds your loader or 3 point.
I was just thinking maybe that it had the wrong filter on it and I then replaced it with a similar incompatible filter and that’s the cause for low flow test. But would that similarly cause the lack of pressure?