1990 SEC, After 1 or 2 minutes, A/C airflow redirects from center vents to defroster outlets. Any ideas anyone???? Looking at the vac circuits, I am suspicious of the 2 check valves. Am I on the right track, or is it likely something else? Thanks for any help to be offered.

More than likely you have a vacuum leak. This will cause these symptoms.

Check all of the vacuum servos on the air box behind the dash.

Thanks, sounds like a good weekend project.