City water

Thanks for all the input.The problem was the water pump.I installed an in line check valve and all is well.

We had the same problem. An added check valve at the pump solved the problem

Simple way to determine which is leaking......fill valve or internal check valve in pump

Remove pump suction line at pump and see it water comes back thru pump

No....then fill valve is leaking thru. Anderson valve?

Yes.then pump check valve is leaking thru. Turn off city water and use pump (complete fresh water tank) and then see if pump still leaks thru when you go back on city water.
As 'DrewE' pointed out.........in-line check valve install on suction line for pump

We don't use the city water.
We fill fresh water tank and use pump
No overfilling of tank
Pump provides all the pressure needed
Pump short cycles indicates a leak.......which I then know about and can find/fix

If it's the check valve in the pump, it will also cause the pump to cycle briefly every once and again if it's left on with no water being used and no city water connected. (Pressure leaks down through the check valve, causing the pump to cycle until the pressure is restored.)

If you put in an external check valve, put it in the supply side to the pump, not the output. If it's at the output the pump will still cycle periodically when there's no demand. It may also be a little bit of crud or something stuck in the pump valve, and cleaning (or just using) the pump may fix the problem.

As rv2go suggests, it could also be the fill valve not closing completely for whatever reason.

If you have the two position valve, one position for city, the other position to fill the fresh water tank, it is possible that the valve in the city water position is letting water seep thru and fill the tank.

I had this happen to me. I have also talked to others that had the same problem

Yes, the pump has a type of check valve.
I would install a standard check valve in the line adjacent to the pump.

NO! When you are hooked up to city water and the ball valve is directing the water flow to all the water plumbing except the water tank. The pump is not even used when you are hooked to city water direct. If you turn the ball valve to the fill fresh water tank position, that is all it is going to do. It will fill the tank until the water comes out of the overfill.

Water pump is only used when you want to pump water off the fresh water tank.

Only way the tank filled up and you say the valve was in the right direction would be that the valve is defective. But ball valves last forever. I have never seen one go bad.