Tractor Repair: 3 point hitch, point hitch, linkage

Question
I have a mid to late 80's model Hesston 5566 tractor.  The three point hitch won't lower.  I can put weight on it (round bale) and leave it over night and it will drop about a foot.  Then it will come up when I engage the lever.  Fluids and filters have all be changed recently.

Answer
I am having one heck of a time finding that model number and I didn't think hesston made a tractor I have seen harvesters'. Anyway you need to check for a knob or lever on top of the housing like under your seat it is called a rate of drop valve.If it is closed then it will not allow the hitch fluid to release. If it does not have this valve then it has to be in linkage or the valve itself. The linkage broken or bent or the unloader valve stuck. If you can give me a tractor serial # then I might be able to look up the model.