bought a ferguson 2 bottom plow and when Hooked it up to my 850 the 3 pt lifts fine. But noticed after a while it drifts down about 4 inches and bounces back up"when the tractor is running". With tractor shut off 3pt will drift very slowly back to ground. Fluid was changed using utf Last fall when i purchased tractor.I was reading my manual and it metioned unloader valve and also check valve. and said speacial tool required. So what are my options and what do ya think i should try first.