[JRTJH]

The only purpose of pushing the backflow valve is to displace any water that "might" be in the PEX tubing between the city water inlet and the "T" fitting where the pump connects to that line. All the remaining PEX tubing will be filled with antifreeze by the water pump while drawing it from the antifreeze bottle.

I always use my air compressor to displace as much water from the hot and cold lines as possible. I connect it to the city water inlet, set the regulator at about 35 PSI and let it purge for about 10 minutes. Doing that will displace any (and all) water in the city water PEX tubing between the hose fitting and the "T" where the water pump connects. So, once you start pumping antifreeze through the water pump, it will "backfill" all the way to the city water connector. There's no need to "push that little white thing" if you use an air compressor to displace the water......