I installed a Whale Y valve maybe 4 years ago for my 1.5" discharge. It has always leaked, I took it apart and cleaned it a few times, same results. If I turn the Whale valve to overboard discharge, a little bit of effluent always gets by (the closed valve) and into the Holding Tank, eventually the Tank fills and I have to empty it. If I don't catch it in time, the tanks gets so full, it swells and then leaks (effluent) out of the deck fill inspection plate on top, due to pressure ... nice. I called Whale and told them about this and they said they had never heard of such a thing. I put a post here a while back and a couple other Boards, and discovered that several other cruisers who had the Whale also found it had leaked.

So, if it were me, I would try another Y valve manufacturer the next time I redo my plumbing. Alternatively, I don't know if anyone makes one that won't leak. For now, I gave up and installed a ball valve downstream of the Whale Y to the Holding Tank, now I have two valves to turn on & off, but now nothing gets to the Holding Tank if I don't want it to.

Good luck ! Cheers, John