The discharge off the pump goes around the vented loop and straight out the overboard through hull. As I close that thru hull when I leave the boat, I don't have to remember to turn a Y valve for a pump out. If I did get the tank pumped with that through hull open, the built in check valve in the pump "should" simply divert the tanks contents to the deck plate. In the worst case the check valve malfunctions or breaks, and the thru hull is open, it will just pull seawater through the deck plate.

At any rate, a Y valve was making the plumbing more difficult to fit into the hamper. the T takes up less space. And it doesn't leak as some Y's do(my old one weeped), a lot cheaper too.

See any problems?