a check valve upstream of the Remote SeaWater pump and downstream of the T. I'd thought of that as a solution, but I generally don't like using check valves, my experience with them is over time they gum up & stick ... not a big deal in this case, cleaning it every year shouldn't be an issue ... and it's a better solution than adding another ball valve that I would have to open & close every time I want to use the Washdown, and to physically get to that valve would be a bit of an issue in my boat under the v-berth mattress.

I'm still curious why I didn't have the problem before, and Vic didn't have the answer either ... I also think the Vented Loop is letting air in, but I wonder why the Marine Elegance Remote Pump (upstream of Vented Loop) is allowing that to happen when the PHII pump did not, also upstream of the Vented Loop ... I suppose mechanics.

thanks, cheers, John