I had the same problem on my 756 a while back. I changed the hydraulic filter and lost prime to the steering and my tractor wouldn't move because the ta is out on the low side. I was told to take the 5/8 inch bolt on the back of the mcv valve(points towards the back tire) and start the tractor and run until oil shoots out. Be ready to shut down quickly. I did this and it picked up prime and the steering worked and the tractor moves like it should.
It will make a big mess when it catches prime. I wedged a bucket behind this bolt with a garbage bag stuffed inside. I had the bucket tilted so it would hold more oil. It worked great...it caught all of the oil.