I'm not familiar with that particular model... but hydrostatic steering in general... either you have a bad cylinder center seal or a bad steering motor. What I would do... turn the wheel hard to the stop on one side, then remove the opposite hose at the cylinder. Now turn the wheel hard against the lock again and see if it drains oil out the line you removed FROM the cylinder side. If it does, I'd wager that the cylinder seal is bad. If it doesn't and the wheel turns in spite of that I'd expect you have a bad steering motor. The seal should be reasonably cheap. The motor will not be.