It is likely one of the brake units leaking on the transmission. They only pressurize when the park brake is released during travel. When park is applied or combine is shutdown the oil supply to the park brake packs is cutoff and the springs mechanically apply the brakes.
Talk to a Case-IH dealer there is a quick test procedure without teardown that will confirm which unit is leaking. The least expensive repair is a reman brake unit forget trying to rebuild yourself. Individual repair parts cost more to repair old unit than the reman unit with warranty and your core returned.
The hydro seal could be leaking but it will only leak when moving not standing still, the brakes will continue leaking with the park brake turned off and combine standing still with the engine running.
If you pull transmission drain plug and run combine standing still with park brake off and oil runs out the drain plug you know it is a bad brake unit. Good Luck.