Not sure how old your tractor is, but it could be a few different things. I had to replace the 3pt cylinder o-ring and backer plate. I have 35 gallons of water on a 3pt carry-all with pressure treated lumber. 285+86+(~)40 lbs = 411 lbs. I can lift it to height, but it will drop slowly, maybe an inch or two an hour with that weight on there. All 3pt's drop, but the quicker the drop, the more chance there is a problem in leaking. Places that leak down would be the cylinder o-ring and backer plate, drop speed valve (not from the controller, but by bleeding around the valve) or even the control valve for the 3pt.
I will say, if you are very concerned about it, you are into it for two solid days of work changing things out yourself, or $600 at the dealer. I made it last a few years before I finally did the job myself. More for the learning of it, than the money, but it was 'work'. Hope you find something easy.