My 1974 Plymouth with power steering has an issue I can not sort out. It had no return to center plus tendency to self steer to the right. I then centered the spool valve and got the power assist to work properly, but no return to center. I then dismounted the steering box pitman arm from the steering linkage and when I tried to turn the box from the pitman arm, it was very heavy to one direction and impossible to turn to the other. (Motor shut off). Then I disconnected the pump lines, and after that it was possible to turn the steering box with reasonable effort to both sides. At this point I road tested the car, and now the steering returns to center. What could be causing this and how should I fix this? Thank you in advance for any insight.