Power steering fluid spills

I 'm working on a 1999 Mercury Sable. The power steering went out when she was out of town. The fluid was good, but hard steering. I changed the pump and I'm getting all kinds of air. It will work and then it wont. I've flushed the system and bleed the wheels with a load and without. When I fill the res. to where its suppose to be it will shoot out of the cap when I shut the motor off. Why am I getting back pressure?