Hesston(fiat) power steering problem

[gwtx]

I have a Hesston 466DT tractor, about a 1985 model. I"ve had it some 20 years, and had very few problems. Recently, the steering will intermittently fail to turn to the left. If I turn it back to the right, and then back to the left, sometimes it goes ahead and turns. Sometimes it takes a couple of turns back and forth before it turns. This can be really scary sometimes. More recently, it has started to fail to turn to the right every now and then, but not as often as the left turn problem. I worked on a lot of things from motorcycles to airplanes,, but never any "full time power steering" setups. It kinda seems like a check valve sticking. I drained it, and flushed it with ATF, and refilled with the specified fluid, but it didn"t help. ANYONE have any experience or ideas ?
Thanks, Gary
 

[showcrop]

Most modern tractors have hydrostatic steering with a "motor" under the steering wheel which activates the cylinder up front. Charland is one of the more prevalent brands. I had an '84 David brown Case that had a problem similar to what you describe. I had just bought the tractor used and they had me remove the motor and take it to them. They tried to rebuild it but were not successful so they got me a new one. They are not cheap.