CDN2USA, I believe all the BX series tractors are the same. I have a BX1870 and when sitting on the tractor, back where my left heel would be, is a round black knob.
In order for me to keep the MMM linkage to stay up, I raise the three point hitch to its highest setting and turn this knob as far as it will go clockwise.
In order to bring the MMM linkage to its lowest setting, while the three point hitch is at its highest, I screw the black knob as far as it will go counter clockwise.
The knob, as I understand it, is to allow for incremental setting points for the MMM. As I don't have a MMM on my 1870 (I have instead a BXpanded under armor), I keep that black knob screwed all the way clockwise to keep the linkage up.
However, when I go to remove the under armor, I raise the three point hitch to its highest setting, unscrew the black knob fully counter clockwise and then when I lower the three point hitch, the MMM linkage also lowers to its lowest setting.
Hope this makes sense.
Ben