280Z center air diffuser question

[mlaw7]

When using the ventilated air or A/C in my '78, I've noticed that there is a LOT more air flow through the right center diffuser than the left. With the blower at full speed, the right side blows nice and strong while the left side is pretty anemic.

What's strange to me is the fact that the air source for both diffusers is the same. Now that I've got the dash out I've looked inside the evaporator housing that sits right behind the diffusers and cannot see anything that would cause this.

I thought this may have been due to a leak in the duct system.

The left duct coming off the evaporator housing fits nice and tight and is in good shape. Also checked the fitting of the short duct and foam wrapped plastic elbow for the driver's side dash vent.

While everything seems to be OK, I can't say it's 100% airtight.

I've also checked the heater vent door on the driver's side. It seems to close pretty tightly but again I can't say that it's 100% airtight.

If either or both of these are leaking air, would that be enough to draw the air flow from the left center diffuser?

Has anyone else had this issue? Anybody have any insight into what might be going on here?