Ford Repair Questions? Ask a Mechanic for Answers ASAP
There is no check valve to keep the fluid from backing into the reservoir, I suspect that you have a leak in the power steering system that has allowed your fluid level to become low, and by running the pump on low fluid is aerating it. First thing you need to do is examine the entire power steering system for leaks, fluid can also leak out the ends of the rack and pinion that attach to your steering linkage, it would be hard to detect this type of leak as the boots on the ends of the rack would capture the fluid and keep it from hitting the ground. As you said the fluid goes all the way down when you start it, the fluid in the reservoir is immediately displacing air that is trapped in the system. The best way to purge this system after you correct the leak, is to attach a vacuum pump to the reservoir to draw the air out.