One other thing to check is if the mower lift knob has in indicator position that says "Top". If so, raise the deck, then turn the mower position control knob to "top" or "lock" if it has one.
If you need to do 3 point hitch work while the mower deck is mounted, there should be some pins to lock it in the raised position.
Good post, but not sure about the part quoted above. In my case with a BX2200, the position knob is what holds the deck in position, when in the TOP position, or any position actually.
In MY case, there are no pins to lock the deck in the top position, since the deck position knob does this.
The 3 PH then can be used while the deck hangs in the position selected. However, if the position selected is not TOP, then I think the deck will lift if the 3PH is raise to maximum. And will lower to the setting chosen on the knob.
Just a clarification, because it sounded like what you were describing is different than the way it works on my tractor.
I remember hearing of a way to pin the 3PH up, but never bothered to find it on my tractor as there has never been a need. I suppose pinning the 3PH up would also lock the mower deck in the up position independent of the position knob.