Thanks.

I actually plan to skip the vented loop in this case. Because the
USCG requires these pump outs to be secured against unintentional use,
I'm going to remove the lever from the seacock. I'm also going to put
the switch for the pump right by the tank so I can watch what's going
on (like if the vent plugs and the pump starts to collapse the tank).
This is for occasional use. I may never use it but I like the boat to
be self sufficient. The idea of being dependent on shore support for
something as basic as getting the sewage out doesn't sit well with me.

A "Y" valve may not be the best solution here either. I may just put
a ball valve in the deck pump out line. The seacock will shut off the
other side of the system.

No, that's a bad idea. Sewage would sit in the overboard line, maybe
forever. With the "Y" valve, I would pump out, then let seawater back
flow briefly to clear the overboard line (everything is below the
W.L.). Then shutting off the seacock and switching the "Y" would
leave mostly seawater in the discharge line and the boat ready for
normal pump out.


--

Roger Long