Zetor Proxima 90 hydraulic lift arms wont lower unassisted.

The hydraulic lift arms on my 2012 Zetor Proxima 90 will not lower unless there is weight on them. When using implements this is not a problem because they have sufficient weight to lower arms. However when no implement is attached or when using the pick up hitch the arms wll not lower without a considerable weight being put on them. i.e. i have to stand on the arms to lower them.
I have checked all the control levers and they appesr to be working correctly. The tractor has been regularly serviced and has done approx 1000 hours.

Hi,

Your tractor should be new enough to have the Mita hydraulics. They don't have the internal lift cylinder, just the 2 large rams on the outside. They are prone to corrosion in the seal area making the rams go tight. If they don't go down when you loosen a pipe, it's the rams.

Mike