Testing Defrost Flap vacuum servo?

[PK Commercial" <pkcommercial@mindspring.com (PK Commercial)]

I notice that I only get partial airflow regardless of where I have the
control head set.  Also, I can visibly see that the defrost flap is not
closing or opening all the way.  I can hear a vacuum leak (hiss) whenever I
have it set to A/C positions, Vent, or Floor heat settings.

My guess is that the vacuum servo for the defrost flap is bad.  I am not
sure how to test the servo itself.  Any specs for it holding vacuum or other
procedures?  Or any other ideas what the problem could be if not that servo?

Thanks,
Phil Kerschner