There is no down force on that tractor or just about any tractor. Most people add weights to a box blade for that. Some of the older IH Farmalls such as the Cub, A, Super A, etc., did have down force. Most tractors do not.
Hydraulic top links are typically used to adjust the angle of the attachment on the fly vs getting off and spinning the top link , which in the case of a box blade could be handy, but to provide downforce like was said, you need to add weights to the box itself.
Are you looking into buying an MT372? Do you have one already? They're good little machines; don't let them not having down pressure scare you away. Not even a brand new machine has that feature.
