You must be on the HAMB too... Welcome to the Stovebolt, we're usually pretty friendly around here. Just don't ask any Ford questions!
I'm not an expert on hydrovacs, but I would imagine if the old check valve works worst case is you'd have a spare if the old one ever failed. The question becomes how do you check to ensure the old check valve works properly.
If you check the tech tips here you'll find a couple of articles on these brake systems, and you might find what you need there. It wasn't until I read Jeff Erickson's overhaul tip that I understood how these things even work.
Hydovac Overhaul If the check valve you're asking about is the one that holds vacuum in the system, you want it to work because otherwise you may have trouble stopping when the engine dies or any other low vacuum situation.
