Originally Posted by Tomguy

This has also been discussed not too long ago here:
power steering pressure build up in reservoir

Basically, it's caused by a leak somewhere in the return area. A leak will allow air into the hydraulic system, which then froths the fluid when it's whipped around in the pump - kinda like mayo except using air versus water as the emulsion fluid. A leak the size of a pin tip can cause this. My steering gear box leaks on the high pressure side a bit, not too badly; and it doesn't foam since the leak is on the outlet vs inlet. I just run low on fluid due to it. You can try the bleeding procedure but I would replace the inlet hose & make sure it's clamped on well first.