7580 4wd-no low on power directr

My neighbor parked this tractor (7580 AC) a dozen or so years ago,with what he thought were power director problems.  I would resurect it if it doesn't cost to much to fix it.  It was being used to pull a smaller air drill about 35 feet and it pulled it just fine, before the problem.                 Now, after getting around fuel bleed problems, it seems to run fairly well.  When I let the clutch out in the low side of the pd nothing happens. pushing down on the high button, and it takes off with a jerk.  If I pusg the clutch in, the low button goes down and the tractor stops, but not instantly.  Power steering works well, but the remotes do not seem to work.   I could not get any pressure with a guage, and the self centering doesn't happen, nor any squeeling from the hydraulics with no hoses attached.  If I pump the brakes they seem to work a little, but not much.   The oil filter light comes on when it is cold, but goes off when it warms up after idling a while.  The rear transmission oil was pretty low . The sight guages are missing with the lower one having a bolt in it, and the upper having the return hose for the air seeder plumbed in.  I added 4 galons and that brought it up to maybe 2 inches low.               So would any of you be able to guess as to what is wrong, and how much labor and how many parts, and if parts are available, and a possible guess to the cost of those parts.   We are trying to figure out if this tractor is worth fixing.  Tach which does't work says about 3200 hours.  Interior is good except for have been in the sun for several years.  I will ask on another topic how it will work on a big square baler.     Thanks   David

Without connecting gauges, I'd suspicion a twisted off low range input shaft.

thanks,   I have a service manual ordered, it should be here in a week.  I assume it will show me where to put the guage, and the pressures to look for?    Would I need a flow guage, as I have none.     Thanks   David