I don't know anything about the reverse osmosis system, but will comment on the shallow well pump. Is your pump kicking back on every 15 minutes without you using water somewhere in the house? Is 40 PSI the upper or lower limit of your pump's pressure switch (i.e., is it the level where the pump usually turns on or off)? If you are gradually losing pressure in the interim 15 minutes, there can be several reasons. You could be losing/leaking water at some fixture or pipe after the pump & tank, though that is usually obvious. There is usually a check valve (one-way valve) just before the pump - if that is leaking, water will force its way back down the well pipe and eventually the system pressure will become low enough that the pump will kick on. Your tank, which is small by the way, could be waterlogged. A waterlogged tank has very little reserve capacity resulting in the pump coming on very frequently. Does the tank have a rubber air bladder inside? You'll usually find a air valve on the tank's side. Take the cap off and depress the valve with you fingernail. If water comes out, you have a waterlogged tank. If it's a rubber bladder type, you can sometime find a replacement bladder or you may have to replace the tank. If its the non-bladder older style, it can be recharged but I'm not familiar with the procedure. I'd check out the tank first. Good luck.