This technical side conversation has been split off of this topic. JasonLion
When the inlet pressure to the filter decreases it means the flow has increased. That sounds counter intuitive, but that is what happens. The inlet pressure at the filter is a measure of the pump discharge pressure and as the pump flow increases the pressure decreases. If you had a valve on the pump discharge you would see that closing the valve would make the pressure rise and opening the valve would make the pressure go down. That's how centrifugal pumps operate.
As the flow though the filter increases the differential pressure across the filter will increase - the differential pressure is the inlet pressure minus the outlet pressure, and this difference will increase if the flow is increased or if the filter gets dirty. The reverse is also true. If you reduce the flow though the filter the difference between inlet and outlet will get smaller and smaller as the flow goes down. If there is no flow though the filter the inlet and outlet pressures will be the same.