Does the combine have a mechanical en electrical hydrostatic,some have been changed to electrical at the dealer. or try ajusting the lock-nut on the hydrostatic handle. if it has the mechanical hydrostatic it's probably rusted.
Get a clear hose 3 feet long about 3/4 inch diameter. Disconnect cable at rear of machine. Slip clear hose over cable for about 4 inches and hose clamp it to cable. Fill hose up with power steering fluid and on open end of hose after you fill with oil, put an air fitting in hose and clamp it and keep constant air pressure of no more than 20psi on hose. Let it set with constant pressure for couple of hours then get in cab and slowly work hydro handle back and forth all the way. This will allow oil to slowly work its way up cable. Monitor the amount of oil cable is using while doing this as you may have to add more oil. Unless the cable is broke, I have NEVER bought a cable when I used this method to free it up. Clear hose works best so you can watch oil work its way into cable.