99 ford taurus power steering problem

I have a 1999 ford taurus (3L,automatic) which I use here on the farm. A fluid leak in the return line from the cooler needed to be repaired last summer now after a couple of months a new problem occurred. When you stop and turn off the engine power steering fluid explodes out the filler cap. The fluid is foamy with air. I replaced the pump with a good used pump, then replaced the rack with a remanufactured unit and checked all the lines. I can’t detect any evidence of a leak, (The old rack did appear to leak).
I suspect that this is caused by an air leak in the return circuit causing air in the fluid which is compressed until the pump stops then expanding back through the pump into the reservoir.
Does anyone have any other suggestions?

Tell your kids to stop putting Mountain Dew in your power steering reservoir.

If it is just a farm use beater I wouldn’t even fix it. Steering will be harder but it will still steer.

If you replaced the pump AND the rack the only thing left is the hoses. You said you repaired the return hose. How did you repair it? I’m wondering if there is a restriction or a kink in it. I’m assuming you didn’t actually replace it.