I traced the hoses back from the Y valve and found another valve at the end.
I opened it and it worked fine. I couldnt tell where it was discharging from though... does it go out below the waterline? It certainly wasnt coming out from where the sink & bilge
So, my questions now are:
1. Where might it be discharging from?
2. Can I leave this valve open all the time? Or do i only open it when I intend to discharge overboard