2000 Taurus power steering problem

Greetings All. I removed power steering (ps) pump when I rebuilt heads on 00 Taurus. Car & steering was fine when I test drove car. When I drove it later, ps pump was making raspy, grinding noise when turning wheels after a few miles, and no ps. Foamy ps fluid. Swapped ps pump but same problem. No leaks found. Any ideas? Thanks for checking!

Did you flush the air out of the system either of times you replaced the pump? Foamy ps fluid is a key indicator of air.

How do you flush air out? I filled reservoir w/engine off, ran engine 5 seconds, then topped off.