Vacuuum is 15 at idle

valve lash will change your vacuum as well, just so you know.
I never pay much attention to idle-vacuum number measured at the carb-base, because there are just too many variables that mess with it, and IMO, it matters not much, cuz vacuum can change in a heartbeat with any change in rpm.
Cam timing changes it.
Ring-seal changes it
valve lash changes it
Ignition Timing changes it
Rpm changes it
A faulty PCV can change it
Even the exhaust system can change it.
And on a slanty, several other things can change it, and
As you have discovered, leaky gaskets can change it.
Happy HotRodding