Help - Bad Check valve? City water filled fresh tank

Had same problem.
Disconnect the city water.
Run the pump for a while and use the water in your FW tank.
The seals in the pump should reseat themselves and function as check valves.
It can be as simple as having the flow pressure clear any minor debris and reorient the seals as designed.
I previously almost never used the pump and had the overflowing FW tank problem three camps. Read here about running the pump could fix it and it did.