If you have flow, low pressure is good. As pressure inside the filter increases, flow through it decreases. The pressure will increase as the filter gets dirty and it is harder to push water through it. On low speed, I have seen systems NEVER have an increase in pressure on the low-speed setting and the only way to know that the filter needs cleaning is to put the filter-pump on high speed to see if the pressure has increased +/- 10# over a clean reading.