Henry, welcome from across the "pond"!!!!!
You have received good advice from the fellows, and if your cub has sat outside or in a humid environment and you don't know the history as stated, it is a good idea to remove the rear cover of the differential, drain and clean/flush it as well.
That will also give you an indication of things inside.
You can use mineral spirits, Kerosene or brake cleaners of the like.
Put on a new gasket and don't overtighten the bolts.
Refill with new fluid and put on a new filter.
Then you will eliminate 75% of most peoples problems,as others have said.
The drive system you have is pretty much problem free, and been used many years.
If you can, shoot us some pictures of your question of a missing relief valve.
It is always possible someone has removed some parts, or "worked" on it, but most times it is the simple things, always best to start simple first.
You can go to
WWW.partstree.com and look up your model and see the specific parts that were installed in your hydro from the factory.
also:
http://www.partstree.com/parts/cub-c...-transmission/
Keep us informed.
