There is no y-valve on my boat. Two large hoses - one hose is waste water in the other to pump out deck plate. A smaller hose comes from the in-tank macerator and goes to the exit thru hull valve. My Egg Harbor had a y-valve in the bilge - pump out or deck plate. My tank is inside the head behind the toilet with a 10 screw cover plate. No y-valve for me.
I took this project on a few years ago after a failed attempt to fix the Grocco in-tank macerator. Nothing works better then a in-tank pump but when they fail its the worst job in boating.
Jabsco pumps are ideal when mounted low on the side of the tank - no lifting of contents, just pump it out. But when it fails its still a mess. On my boat there is no side access to do this.
Instead made a new cover plate and mounted the pump above the tank as close as possible. I made a "honey dipper straw" of 1 1/2" PVC to go into the tank to connect to the new pump. All hoses are blue silicone, expensive but easy to take apart if you have to. Jabsco claims 5' of lift (self priming) but you don't want to go more than a foot. If the pump doesn't prime quickly it will eat the impeller.
Some of my best work is covered up and never seen again if I did the job right. I take pictures of all of it! I'll post a photo of it later. So far its been working great!
09/21/2021: I have been having a problem getting the pump to prime after sitting for a long while. Put some water in the hose and it will pump out fine. Modified the install with a 90 degree reducer 1" to 3/4" and installed a 3/4" vented loop. It more about keeping some liquid in the hose at the pump. To winterize I'll have to pull the 90 degree hose, dump out and use vegetable oil - not sure if the impeller will tolerate the alcohol in the pink antifreeze.