I have a 1966 ford 3000 and an early 1970's 4000su both suffering the same problem. when I have an implement hooked to the 3 point hitch they both lift very easily, but there is resistance trying to move the lever up and it immediately drops when I take my hand off. the heavier the implement the worse the problem is but nothing I hook up is excessively heavy, typically an old 2 row cultivator or something similar. The lever moves very smoothly and stays where put if tractor has nothing hooked up. I have owned about 8 of these tractors and these are the first 2 to have this problem unless I have something very heavy hooked up. Any Ideas?