I found it. It is a vent to the cabin.

According to the Peter Schmid transmission website that someone was kind enough to send me, I find that this line is merely a vent to the cabin. So it was never supposed to hold a vacuum. What was the purpose of venting in the cabin, I have no idea.

So now I have no vac leak. However I wonder if the top Y at the proportioning valve (end of injectors) is supposed to hold only 5 lbs of vac; if I pump up the mityvac, it drains down to 5lbs and then holds there. If I unplug the Y from the proportioning valve and test the Y and the lines, they hold 15 or so lbs. So the rubber Y and the lines are for sure good.

Getting there slowly...