The city water connection has a CHECK (one way valve
) built into it. Apparently, for some reason, the CHECK is not seating.
With pump OFF, remove the screened washer inside the city water connection. Push on the valve a few time to try and make it seat in place. Most of the time, this works.
If this doesn't work, connect a water hose to the city water connection and run water through it. This may help flush it out(if anything is obstructing the valve). Remove the water supply hose and try the pump again.
If this doesn't work, you may have to replace the city water valve.
If you're camping where you have no access to a water supply, you'll have to pinch off the hose(from the pump to the city water connection) to stop the flow. Use vise-grip pliers with a softener between the jaws or a "C"-clamp, or whatever to stop the flow.