hst leak

I have a 2005 BX23. For quite some time it has been leaving a little trail of hydraulic oil that drips off the bottom under the HST. I have searched to no avail; I finally found a mechanic who understands small Kubotas and he spent the time to find the problem. There's an opening on the top...

My valve is leaking around the piece that is circled in yellow. Any ideas what the fix is?

My BX1500 leaks HST fluid slowly. Is there a UV dye that is compatible with Kubotas HST fluid? Thank you