The white ring (washer) comes that way, it has to have a split in it or you can't get it on or off.
I got new rings and washers for one of mine some time ago, I layed them down some place and can't remember where I put them?!. Guess I'll run across them sometime... I think they were kinda spendy but not sure exactly how much. Do what you want, but these are one of those things that I wouldn't try to get at an auto parts store.
There are also "O" rings inside the valves, these are what cause them to leak around the stem. You can try to "swell" these rings with brake fluid, repair the valves, or replace them. Someone here knows where you can get the valves for a decent price, but I'm not him (maybe he will tell us again). Here are a couple of links that show how to repair the valves.
http://cubfaq.com/relief1.html and
http://cubfaq.com/relief2.html
As far as the Trunion problem you are having, here is a link for that (do a google search for "Cub Cadet trunion repair" and you should get several other decent hits).
http://cubfaq.com/trunionrepair.html
If you do that then you will need
http://cubfaq.com/neutraladjust.html
The "Correct Police" should be along soon to discredit everything I have told you so watch for his post.
Good luck and keep us posted.