I have just replaced the drive belt on my JD and the belt has no tension at all when the tractor is off. I have replaced belst a few times and each time there was tension on the belt. As an example previously when I would turn the blades by hand I could move one blade with no problem but to move the second blade I had to hold onto the pulley to prevent the first balde from moving.
I have triple checked the belt routing and that is fine. I adjusted the brakes thinking this would tighten arm holding one of the pulleys but this didnt work.