I just rebuilt, with help from another local club member, the steering with basically all new parts from intermediate shaft through to the wheels.
The car now turns easier to the right than to the left. There is no air in the system, and the effect is the same when the car is on the ground or in the air. It also seems slighty easier to turn to the right than the left when the engine is off (and the front end suspended), but this might be my imagination. It also doesn't self-center, especially from turns to the right.
My suspicion is the steering box, but before I put the old one back in, I'm looking for other suggestions. With so many parts replaced, what else could cause this problem?