How to Indicate A Vise in Just One pass

[Wawoodman]

I tried it yesterday. The problem I'm having is that my needle runs true for part of the way, and then moves. And sometimes it seems to work fine, but when I go the other way, it's way off. So it's one of two things, to my mind: either the jaw is not truly flat, or my indicator (Starrett Last Word) is sticking. Both the indicator and the vise are eBay purchases.

If I remove the jaw, is the vise body accurate enough to work from? If it's the indicator, can it be serviced economically enough to make it worthwhile? Or should I be looking for a new one?