Normally Aspirated Skymaster Cabin Heat Shutoff?

[rhurt]

Has anyone experienced problems getting the cabin heat in the normally aspirated models to completely shut off? Has anyone rebuilt the heater/ventilation control airboxes?

The changed the design of the control airboxes around 1970, and I wonder if the earlier design was prone to leak and if the later design solved the problem. Either way these things look tough to access.