On a 1990, the secondary system pressure switch is located on the bracket under the ECM. You can test it by disconnecting the vacuum line, connecting a vacuum pump and then use a ohm meter to verify that it operates at the correct pressure setting, (which I don't remember but it is in the Service Manual.)
If you have access to a Tech 1, there are very good tools available to verify the operation of the secondary system. I see that you are not too far away from WAZOO. If you could get over here, we would be glad to help troubleshoot your Z.
If you are convinced that the secondary system is holding vacuum and that you can see the secondary actuators move in the open direction, I would move to troubleshooting the secondary injector relays and the secondary injectors wiring.
Jim
jim@gizmosart.com