jcbj,
You're not getting any replies to your post, probably because it was a little vague.
If my Googling is correct, your Mitsubishi D4000 is a diesel 45HP tractor with a gear (not hydrostatic) transmission.
You say it "coasts" when going downhill. I'm not sure what you mean by that.
Do you mean it drifts when parked?
Or it free-wheels out of control going downhill?
Could an implement (like a brush hog) be over-driving your machine giving you the impression that you are coasting?
Give us some details, and someone here will probably have an answer for you.