Are you sure you need to replace them? Ours wouldn't close completely and we thought the same, but a serviceman at a local RV shop said they are probably gunked up. He recommended Thetford RV drain valve lubricant. We followed the directions and they now open and close completely!
2011 Bighorn 3055RL, 2011 F350 DRW 6.7L 4x4 Diesel Lariat and Hensley TrailerSaver BD3, 1992 Jeep ZJ and 1978 Coleman Concord Pop-Up for remote camping
Dave & Renee plus (Champ, Molly, Paris, Missy, and Maggie in spirit), Mica, Mabel, and Melton