I had the same question and went ahead and installed a new anti siphon valve when draining my 1999 200 Osprey tank for the off-season at the (stern) advice of my marine mechanic. He did install one made of materials that he said were less likely to corrode and stick. He also noted the tank shifted when empty and suggested I look into that, and find out how it is secured and get ideas for how to fix it, but having posted that subject here, and asking for advice, for some reason no one has replied.