I was working on my 86 and started the engine and turned the wheel lock to lock after messing with a turn signal cancelling issue (left signal won't cancel) and I noticed that the steering was tougher than usual and the pump was whining. I went to check the fluid and it was pouring out of the cap and was all foamy. I let it settle and cleaned the engine bay up and started the engine again without the cap on and it seemed fine. Any ideas?

 

I let it sit until the fluid was no longer frothy and started the engine and turned the wheel lock to lock as if I were bleeding the power steering system and it seems ok. Fluid is nasty and will be changed this weekend when I'm off work. As it sits now:

 

The only way I could see this happening is a blockage on the suction side causing cavitation in the pump (whining). Then as the fluid cycled through the rack and into the reservoir it overflowed due to fluid not being drawn into the pump. You had all of the systems fluid trying to fit into the reservoir. After all of the fluid was out of the rack the cavatized fluid remained which is the frothing you saw. Can't speak to specifics on if it could be the filter, suction hose, or debris inside the pump.

 

Time for a removal and cleaning of all of the lines and the pump and some fresh fluid.

Also noticed this hole on the resivour cap. I assume its a vent hole. Never noticed it before today: