Hi,
I blow the lines out with this
type of plug with the compressor set at 40PSI . After I run RV antifreeze through the system, I will go back out to the city water connection, remove the debris screen and push the check valve in with my finger for just a second. Your system will be pressurized so watch the spray back. You will see antifreeze in the spray back from the city water check valve. Don't forget the outside shower.
crunchman