One possibility is that there is air trapped in the system. The proper way
to bleed it out is as follows.

1. Start with a cold engine that has set up without running for at least
several hours.

2. Disable the ignition system by disconnecting the wiring harness at the
ignition coil.

3. Jack up the car so that both front wheels are slightly off the ground.

4. Make sure the P/S reservoir is full.

5. While cranking the engine turn the steering wheel all the way to the
right.

6. Recheck the P/S reservoir fluid level and refill if needed.

7 Again crank the engine and turn the steering wheel all the way to the left
while doing so.

8. Recheck the fluid level.

9.  Repeat steps 4 through seven until there is no change in the fluid level
between steps.

10. Reconnect the ignition coil and start the engine.

11. Turn the steering wheel back and forth several times.

12. Turn the engine off and lower the vehicle. recheck the fluid level and
fill to the full level if necessary.

All the air should now be bled out of the system.

Another possibility, although rare, is that the pressure hose from the pump
to the steering gear is swelling under pressure when the engine is running
and collapses when the engine is turned off. That could cause the blow back
of fluid if the power steering pressure valve in the pump is also stuck
open. You should replace the hose and the pump in that case. Then you will
need to bleed the air out of the system after that repair.

In some rare cases the  fluid can become aerated which makes bleeding it out
almost impossible. In that case you would have to flush out all the old
fluid and replenish with new fluid before bleeding again.

Several years ago Ford came out with a TSB that instructed technicians to
use a special reservoir cap that could be hooked to a vacuum pump which
would help break up the air bubbles and make the bleeding process more
successful, but I found it easier and better to just flush out all the old
fluid and start fresh.

hope this helps.