Thanks for your input to this issue. I had both the 8 - 10 connector plastic vacuum manifold replaced and the vacuum storage tank pressure tested. The vacuum storage tank I'm told passed the pressure test (whatever that was) and the vacuum manifold had a slight crack in it. After replacing it, the issue stayed the same.
In testing the fresh air vacuum diaphragm, how does one go about testing it and replacing it, since it has a high failure rate. I'll try playing with the EC button & see if the vent problems go away. I'll post the results.