Jeremy, Theobald: try the FAQ link at the top of the page, I think they cover relief valve diagnosis and repair. I know the topic has been addressed in previous posts, you will probably need to use the search function and chase down some threads. There is a lot of good information on this site, but sometimes you have to dig for it. Also, from the main FAQ site is a link to Charlie's FAQ page which has a lot of good information on it. Finally, the Parts Lookup for your model will turn up an exploded view of the transmission that will help you identify which parts the posters are talking about it.
I think the relief valves can be removed, and cleaned, but if that doesn't work, you're probably looking at replacement, unless you're a machinist are very friendly with one.
Good luck. Charlie's right, every Cub has beauty, but some of them need their makeup on first.