What causes foamy power steering fluid on a 1990 Honda Accord?

I just had this same issue on my 2002 V6 accord. It gets foamy if air is leaking into the Power Steering system, or there is a kinked hose. My problem was solved by replacing the little O-ring on the inlet to the power steering pump. At the Honda garage, it was $1, but they wanted $100 to install it. My local gas station mechanic installed it in about a half hour at a cost of $30.