D-17 tractors were designed for two lifting modes. One is lift and hold. In this mode it lowers in all the way down position and lifts in the all the way up position. Somewhere in the middle it holds. The other mode lifts to max and stays there if the lever is all the way up. It will start to drop as soon as you move it down a little bit. But it should hold all the way up.
I recommend you get a manual that describes these adjustments. It will help you decide if anything is wrong.