In the Economy Mode with selector wheel set to Max there should be no air coming from the center vent.
Also in Defrost Mode, no air should come out of the center vent.
If you do, you may have vacuum pod, switchover valve block or push button unit problems. The pods crack with age, the switchover valve block suffers from solder cracks and the PBU can have a variety of problems.
I would see what is causing this before you throw money at a new monovalve.