The whole unit is sealed, so you will need to replace it. Here is a video on how to remove it. speed up to 8:40 to see what the assembly looks like when it's removed.

https://www.youtube.com/watch?v=9WDW4lyIFjo

[IMG]http://i130.photobucket.com/albums/p265/migsbig/Hippo_skull_dark-1.jpg[/IMG]