That y is the fusion of your fill and drain connected to I believe your vent of your "front" pnp. This is so if you turn it on it does not flood boat. One piece coming from rear has check valve that is your fill, 2nd hose from rear of locker your drain. I (dealer with a warranty repair for leaking overflow during operation- never fixed that issue but relocation was on factory)relocated my vent to the side wall by driver foot well(maybe a mistake) so I can't remember where your vent leads to. We also moved fill line to same location in foot well instead of rear bottom of locker sharing binding space on mushroom bag. The mushroom bag when really full tests those plastic high flow connecters and the plastic threaded bag ports. In rears to stop hose kinking we moved valves to a 90 change to straight up and down in the outside front corners of lockers with handles stuck sort of in on position-installed a 90 elbow from Lowes plumbing (again on warranty cause kinking issue ). And drain cut to barely sticking out of box and bottom vent cut to bag vent hole placement I imagined when full. I cut hose lengths in rear lockers and installed quick connect high flows from wakemakers in all three lockers , dealer solved kink by rotating valve and installing 90. For days we could go on, but that front compartment (I have chillax seat making it a bit more complicated cause chillax hardware
750'rears well burped
950 mushroom well burped
Full fuel
Wedge
Stock ballast
And wife and batteries and ice chest is our setup minimum
The wife on wave Sunday iPhone high def full screen she is 5 3" and stays away from boat and this is 80% magnification
https://www.facebook.com/daves.wakeboat ... 812466861/I'm tired of all the typing this is going to take . Know this well. Based upon your your question I can help you more on phone. 504-430-2563