I'm mostly certain I know the answer to this one, but I wanted to check it out with you experts.

I got my first vintage pen, a "51" vacumatic that was an ebay purchase. I was told to expect to have to replace the sac. When I depress the plunger with the pen's tip in water, no water is sucked in when the plunger is released. This means I definitely need a new sac, right?

Stephen

Most likely. You need a new diaphragm (that's what Vacumatic sacs are called) in Debutante size.
You can find one and other useful stuff in the Tryphon Catalog.

Tryphon Catalog

Stephen, you will also need a Vac tool for removing the filler and some patience.

I suggest that you check out Richard Binders reference info for a lesson on how this is done. It's not hard after you have done a few, but you DO need the correct tools.

As for tools, see Giovanni's site.

Ususal disclaimer here, no affiliations just a very happy customer of both Gentlemen.

Cheers,
SG

http://www.richardspens.com/

Thanks. Actually, I'm going to take the easy way out and just send the pen to Richard. That way he can look at what else might need to be done to it.

Stephen