Why would you use the old o-rings ?
You can get adaptor kits at Pep Boys quite cheap and the new ones are
bluish-green and work with all the oils you may encounter these days whereas the old
black ones wont.

I would suggest that you pull the vacuum down for at least 45 mins.   Longer
if you can wait.
If you are using a conventional gauge set , then you can open both high and
low side to the center hose, assuming thats where you have the pump attached.
Both sides will show a vacuum reading and I would like to see at least 28.
What kind of pump are you using?   Sounds like a medical pump.   It maybe
worth your while to borrow an automotive a/c pump which probably wont give any
volume values.    Once a good vacuum is pulled, I doubt if the volume means
much.
I borrow a friends'.  I dont know what brand it is, he just calls it Monica,
lol

I have worked on 3 systems this spring, including converting my recently sold
'84 Westy to R134a.       If you want to chat on the phone about it, I would
be happy to.
p-mail me for the number.

good luck,
Steve Cotsford
Columbia SC