I should have mentioned, there is a completely different approach which I like better, in some ways. The system I had which was converted, I DID NOT install R134A adapters on the system. I don't remember why, it may be some of the issues you are having.....they didn't fit correctly.
Anyhow, when I charged the system, I took a low side adapter and screwed it onto the middle port of my gauge manifold. Then just hooked the manifold to the old R12 flare fittings. This allows connecting an R134A can to the manifold, "easy."
