The internal check valve in the city water connection has failed....they are just a plunger, spring and o-ring.....cheap plastic junk
Just get a 3/4" PVC plug and screw it into the city water connection when you are using the on-board pump.
IF you can not remove the city water connection.....you may have to replace the whole assembly/housing. The PVC plug will work just fine until you locate a housing OR forever....well at least for a really long time :B
Although WW went out of business in 2009........it has reemerged so you may have luck contacting them.........
LINK