Tractor Repair: Backhoe transmission, clutch pack, case 580

Question
I have an 89 case 580 Super E that slips in forward gear but not in reverse.  This happened two times before and each time the filter was plugged with black material but after cleaning the filter it was fine for awhile.  This time the filter was not plugged so I thought that the clutches must have failed.  I pulled the tranny, tore it down and inspected it.  It looks like new.  I discovered that there is only one clutch pack and they look perfect, no wear and no hot spots?  It also started getting hard to move the lever into reverse when it started slipping.  I don't know if it is related.  Any idea what could be the culprit?

Answer
Well it has been along time since I have rebuilt one of those shuttles. But you might want to look in the valve. Make sure all of the holes and passages are clear. It being hard to move into reverse might be the balls and springs on the linkage. Where the linkage hooks up is a plate and the ball locks the handle into gear make sure all that is ok. Other than that it has to be in the valve or in the passages.