If you operate the steering without the pump running (as you would if the belt got thrown off) it pushes fluid up and sucks air into the rack. Ford says to apply vacuum (not from the engine) with an adapter to the top of the power steering pump, where you pour in the fluid...

But i've found that if you hold RPM at 2500, and steer the wheels lock to lock about 20-30 times, it will do just as well.

Good luck!