I also had the fresh water tank overflow. I just fixed the Anderson valve about a month prior so I added an inline check valve to the water pump as danemayer suggested. Been fine since. I carry plumbers grease for the Anderson valve. If the valve starts to require a lot more force to turn than normal, I take it apart and grease it. Only had to do that once so far. You can order and replace the check valve that is in the water pump, but when on the road, adding an external check valve is quick and easy.
Marc,
In a call last week, Anderson brought up the subject of lubricants. They were very concerned about users lubricating the valve because petroleum products will damage the seats and o-rings. They only lubricant they support is
Dow Corning Molycote 111.
How serious a problem is it if a different product is used? I don't know. I can only say that Anderson Brass has communicated that in their view, it's serious.
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Putting the check valve on the suction side didn't work for me for some reason. I had to put it on the output side. This was my second shurflo to have the check valve go bad so I went to another brand of pump.
Installing the check valve on the output side of the pump will stop the unwanted filling of the fresh tank. But, it's possible you may get undesired effects depending on how much water is leaking through the pump's check valve.
When you close the faucet (while pumping water from the fresh tank), the pump builds up water pressure in the pex tubing. The pressure sensor on the pump is then activated, shutting off the pump. But if water then leaks backward through the pump, the pressure drops and the pump starts up again. The cycle time on the pump is a function of how much water flows backward through the pump.
If you experience unwanted cycling of the water pump, you may want to revisit the positioning of the check valve.