Some Cub Cadets had relief valves that were automatic, some had a manual relief lever. It's hard to say which you have, since so many have been worked on and/or swapped in the last 30 years.
Since you replaced the hydro maybe you remember the two valves on the top of the pump. If they had little buttons in the center, then it requires the manual relief lever/linkage. If not, it should roll when pushed, not easily, but it should move.
If you have the manual valves but no lever/linkage it won't move. There is no rear end/differential release, just the hydraulic valves.