Hydrostatic Power Steering problems

This is off color (Massey 255), but I was hoping for some input.  The tractor had been setting a long time before I got it.  Previous owner said he thought a spindle was locked up, so had started taking the front end apart. When I jacked it up with the tie rods undone, I can move each wheel just fine.  They are a little stiff, but moveable. When the tractor is not running, the steering wheel spins freely.  When it is, it turns very hard especially to the right, even with nothing hooked up to the steering gear.  I cleaned the screens on the powersteering pump intake, no difference.  I thought it might have an air lock, so I opened the fittings on each end of the steering cylinder, and had oil coming out of each fitting without turning the wheel.  That made me figure the oribal motor is shot.  Does that make sense?  I've had plenty of them cause trouble on AC tractors and had seals go out, or they would turn freely with the tractor running and not turn the wheels, but I havent had it go bad this way before.  Before I spend the $350 or so a new one would be, anyone have any ideas?  Thanks in advance. 

Not being familiar with the MF hydraulic system(open center or closed center), why don't you put a plug at the end of each cylinder hose and then if the orbitrol is bad, I'd think it would still turn somewhat easily.....orbitrol good?? ..then the plugged lines would make it turn hard if at all. You also could be suffering from a pump delivery problem, which should allow turning of the wheel when lines are plugged. And by the way, in nearly 40 yrs of chasing AC's, I've NEVER replaced an orbitrol, just resealed them occasionally for leaks (usually up the steering column tube).

I don't recall when but sometime after the orbital steering valves started being used they began using back pressure from the valve to help dampen the action of the steering cyl. Light back pressure keeps the "drift" from occuring also.

Frothy oil sounds like sucking air through seals at pump.Never work right like that.

Thanks for the replies everyone!  I unhooked the lines from the steering cylinder and hooked an air line to one end and did not have any noticeable air travel, so I put gauges on each hose going to it and started the tractor and the steering wheel was absolutely locked.  I couldnt move it either direction and showed 0s on both gauges.  So, I took the output line off the pump and had spotty flow with pressure I could nearly stop with my thumb.  I took the pump apart checked and cleaned screens and the bypass/relief valve, put everything back together and had the same results.  So, it looks like I am looking at rebuilding the power steering pump  or getting a new one instead of working on the orbital.  Thanks again guys!