PSI for Showa airshocks on Nighthawk???

Hi. I have a 92 nighthawk 750 that has aftermarket Showa air shocks
on it. The guy I bought it off didn't even know they weren't stock
and was totally unable to help me. There is a tube running from each
shock which I presume is an airline, and they meet up under the seat
at a little air valve (like on the wheels). At the moment, there is
no measurable air pressure in there. I don't know what the bike
"ought" to feel like, but I'd like it if it was a little stiffer back
there. I've been told it's just add air until it feels right, but I
don't know what sort of window I'm working in and I don't want to hurt
them. What sort of psi is a limit? Any guidelines? Thanks.

Luke

PS don't email a response, I don't check that email. Thanks.

 

Well, turns out there is no air pressure because they leak. I filled
them up last night and almost immediately there was nothing again. Is
this fixable? Do I (can I?) need to rebuild them, or do I just toss
them? Is it worth the cost of new airshocks or should I replace them
with OEM springs? Thanks all.

Luke