This is a typical diagram of your water system. You can see the one-way valve between the PUMP and the City Water port. This is suppose to prevent the water pump from sending water out of the city water port when it is being used...
Sometimes you can just stick a small object into the city water port and exerise the one way valve flap... It probably has picked some debris keeping the flap from not sealing closed.
The other idea is to just hook up a screw on water cap that closes the city water port off...
Roy Ken
