I have an FX6 and an FX4. I usually take the Aquastop valve apart and lube the special o-ring with a small amount of vaseline or petroleum jelly. I do not remove the special o-ring, just gently apply some to the exterior. There is a tab on the back side of the valve. Gently pry it over just enough to let the valve come out. Here are pics of mine apart. You can plainly see the tab and special o-ring. If your special o-ring is broke, just as cheap to buy the whole valve.
Pic #1 - Looking down into valve open.
Pic #2 - Looking into valve closed.
Pic #3 - Looking at back side of valve where the tab is. (I've already pulled the valve out.)
Pic #4 - Shows the special o-ring that you can simply apply a small amount of vaseline to and remedy the dry/sticky hard to turn valve. It also helps make a tighter seal.