gary,
I have the same Hynautic system on my boat. If the dial has rusted off on your gauge you can just take the pressure reading off the valve with a regular tire guage. I imagine that you could replace you're broken gauge with one from an auto parts store, but I haven't had a reason to try that yet. 25 - 30 lbs is the recommended pressure (at least thats what the sticker says on the side of my canister).
Here's some more information about bleeding the system:
http://www.seastarsteering.com/PDFs/182000-A.pdf