I am replacing the holding tank on my new old boat and the new tank's bottom will be a couple of inches above the water line. I want to be able to discharge by gravity but there is a perfectly good macerator pump teed off of the pump out line about even with the top of the tank. There is a Y coupling at the seacock so I could put the gravity line in one side and the macerator discharge in the other. My question is, if I tee off of the tank discharge to the pump out/macerator line and the gravity line to the seacock, do I need a valve between the tank outlet and the seacock. Will I wind up pumping round and round when I use the macerator?