The control valve and release valve stem seals wear and leak. I don"t know if Case has seal kits still avail.
The body of the depth control block is 2 halves, and if original has a lead gasket that will leak if the 2 halves vibrate loose. Probably won"t be able stop the leak by tightening the fasteners. Replace with a regular paper gasket material.
Go to this link and read a little. Click on the pages to enlarge for reading.