Johnson Mini Aquajet Fresh Water PumpThis is an ideal pump for a small pressurised water system in a boat, caravan or RV. Working principle. As the pump runs, pressure builds until reaching 2. 8 Bar or 41 PSI. At this point, the pressure switch cuts the pump motor off. The pump is equipped with checking output valves which hold the pressure internally. When there is a water demand, the moderate pressure drop switches the pump on again. The result is that when you turn a tap or shower on,