There is a check valve in the pump head. Mine stuck open and while using the city water it would fill the freshwater tank. I pulled the pump, removed the 8 screw to the head mounting, and just touched the check valve and it snapped closed. I have not had any trouble since. There is a diagram on this pump model to show where the check valve is. Took about 40 minute to fix and most was removing and replacing the pump assembly in the trailer. Tight space in there.
The pump is a surflo 4008
2015 Jayco Jayflight 24FBS
2013 F150 Super Crew 3.5, 3:31
2004 f150 Super Crew 5.4, 3:73 traded