Why the gusher of steering fluid?

My son Aaron and I finally got his "DARTSTER" on the road this weekend for its maiden voyage. While we were tying up a few loose ends with misc. items, we needed to install the plastic splash shields at the back of the front fenders. To gain access I had him turn the wheels sharply to one side. The Dart has power steering and I had just topped off the fluid level to the "cold" mark before we took the car for a spin. When he cut the wheels to one side, a gusher of power steering fluid dumped out the fill neck.

Is that typical for over-filling (and the dipstick mark means nothing), or is it a symptom of something more serious? I looked down inside the pump and the fluid level was down below the cast-iron-looking curved surface that's below the bottom of the fill neck. I topped it off again, not wanting to run it with low fluid.

Any ideas?
Jerry