Jon Kraus wrote:
We determined that what probably happened was that the extended period
of idling on the ground caused the AI gyro to spool down some. It took
several minutes at cruise power for it to come back up to speed and
after that it indicated correctly.

You should be getting full vacuum at normal ground idle speeds (1000 RPM
or so). If your gyros are not spinning up in the time it takes you to
taxi to the runway and do a normal runnup, something is wrong with the
vacuum system or the gyros.

In any case, congrats on a job well done.