how should I prime my old walls?
Painting new plaster is much different than painting old plaster, mainly because of the chemical reactions in setting plaster and the long time-frames it can take to completely cure.  70 year old plaster is much easier to deal with, although you are going to want to prime it. I use Kilz latex (not a brand recommendation - other manufacturers make a comparable product), but any high quality primer should be fine.  Drywall PVA sealers aren't necessary on plaster unless you use drywall mud or spackling to make repairs because plaster is much more water resistant on its own.  If you patched with something like that, let them completely dry and just spot coat with PVA sealer.