Mark T is out on vacation so I will be taking on his questions while he's away. I've been over the information you've provided for the situation on your tractor. Notice around each pulley there should be a belt keeper. If the belt keeper is missing then the belt is going to fall off when you disengage. Each pulley on the deck has a belt keeper, so if you do not see one at each pulley, it's missing. If you do see them, it's possible that they have been pushed away at some point (belt jamming will usually cause it). The keepers should all be almost touching the pulley. It should be very close but not touching. If you can move the belt easily between the keeper and the pulley, it's too far away and you'll need to be bend it back into place. Let me know what it looks like and we can proceed if necessary.