When driving the Edsel in or out of the garage, I found oil on the floor that came from somewhere near the power steering control valve. The leakage got worse, but I couldn't find the cause.
By the way, the clear drop is water from a previous car wash. Annoyingly, when I was first searching for the leak, the ATF was spread in a way that made both fittings look leaky. Finally, with the help of a friend, the real cause was found. After cleaning the lines, a drop slowly emerged upon the lower line. The crack was so small that it couldn't be seen or felt. But a close up photograph makes the damage visible:
Luckily those lines are standard brake lines, available in different lengths and diameters, ready for installation. I only had to bend the spare part to make it fit between the control valve and the power cylinder.