Ok, switched to seawater, tried before diaphragm r/r but had seacock closed. Opened and it works. Definitely the freshwater side. Appears line from the tank to the shutoff valve. Does anyone know what that special brass fitting is? Perhaps a filter?
My guess is it is a check valve to stop possible sea water from entering your fresh water. They can get stuck open & closed. Close all valves in line with it and unscrew the top plug to check the valve.
Update. Opened check valve cap and it appeared free. Closed Back up and opened valves and it started working. I’m guessing an airlock or the valve was stuck closed.