What a very nice Comet... although it's a little young for my taste (that's a 65 v 64 joke
With regard to the Power Steering: unless there is something radically different between 65 and 64 (I don't think so) and if it's original, there really isn't any bleeding required since the return line goes to the reservoir and air escapes at the top of the system. too much pressure in the system for air to stay in there. That said, here are a few things that could impact the steering in my experience.
1) the steering cylinder is bent (the ram attached to the drag link that pushes and pulls the steering left and right) or the bracket it attaches to (bolted to the driver side frame rail) is loose or misaligned.
2) the idler arm bushing. If it's loose, torn, or slipping, it may work fine to on side but slip in the other direction. That thing is a bugger. 65 may be better than 64 but it's still an issue.
3) the power steering control valve. That thing is a miracle of science and if somebody told me they had rebuilt it, I'd want to know more about their attention to detail, where they got the parts kit, etc. If anything is wrong in that spool valve it could have all kinds of affects.
4) that said, i seem to recall, there is an adjustment for the control valve that sets the tension of the back and forth of the spool valve, so that may be the easiest thing if the PO didn't set it correctly.
5) If anybody rebuilt the steering box, that could be another issue. When rebuilding it, it's tricky to get all those little bearings together and recirculating correctly. if one of those buggers gets outside the loop, the box will work but it will bind one way.
6) a dozen other things, but these are the first ones that come to mind.
Easiest thing is just lay under there while it's running and have somebody move the steering from side to side. look at each of those pieces and see what's up.
This is a great site for learning everything you want to know about that PS system, and if you need parts, etc (and if he's still in business) Randy is a fountain of knowledge.
http://www.stangerssite.com/
Sorry of this is TMI, but I spent a good chunk of time rebuilding one of those systems once, and it's one of the few times I get to put that useless knowledge back in action.