Self-centering of power steering

Triumph Stag 1977 51000 3000cc V8

Problem: Had a reconditioned steering rack and track rod ends fittedby a specialist garage. But. At speeds below 20mph there was no self-centering at all, so if I turned a bend, there is a horrible and scary lurch if I did not center up.

I decided to check the fluid level in the pump, and according to the level marker on the cap, there was way too much fluid put in. I removed some of the fluid, so it is spot on the level marker and went for a test drive. Well it seems alot better now, a little bit lighter and it self-centers.

Question: Would too much fluid cause the self-centering problem, or is it just a coincidence that it feels better and self-centers now some fluid from the pump has been removed?