Power steering noise

After about half hour of driving. It starts the usual moaning. Have replaced fluid many times. If I pop the cap after driving with the van OFF lots of bubbles. Fluid is over the Min line and the there is almost 1/2 inch of bubbles in the reservoir.

I popped the cap few days ago after driving and cranking steering one side to the other and the checked the fluid when the van was running and the fluid was bubbling (As if boiling!)

Btw used Lucas power steering stuff and so far so good. No more bubbles or whining.