I inherited an old 706...i don't recall ever seeing a rate descent control knob like is on my Kubota.
I am pretty sure it's all tied into the draft control system. The draft control is the outer lever, next to your 3pt control. Fully forward should be off, then it goes from light load to heavy.
I suspect if operating your draft control isn't making any difference, you will need to rebuild the draft control valve. Perhaps there is an adjustment on the valve itself (not sure).
You should see a link to IH tractor parts - here you can pull up a parts schematic and see what's what. Since I am on my iPhone, I can't load the pages..but I have used this site in the past to troubleshoot (and order parts). Hope this helps.
NOTE: I edited my parts link..for some reason my local supplier's parts link no longers works...but this one does!