weird steering

If you have purged all the air out of the system, it sounds like the spool valve is not centered. Get a manual that details how to center spool valve.
Many years ago I rebulit a p/s box and failed to get the s/v centered and the wheel did the same thing, all the way to left on its own. The s/v is located under the hose fitting on top. Its been a long time ago and I don`t remember how to do this, but its not to hard.