I saw the post you made last week, and was skeptical of the vent theory all along. However, if you would like to chase this lead, I can tell you where to find the vent. It is in back of the tractor on the lift cover. Specifically, it is one of the bolts that holds the draft spring retainer in place.
If you look at your top link bracket, find the pin that attaches the top of this bracket. This pin is attached to your draft rod. Behind that you will find a cast housing which contains the draft spring. This housing is retained with 3 - 7/16 inch bolts. One of these bolts is drilled hollow all the way through. This is your rear axle vent.
Let us know what you find.