OK,

I blocked off the hoses and took off the top of the mono valve. After I got the slug out, I checked the diaphragm for cracks and such. Everything looked fine. The screen didn't have much crud in it either. In fact, the whole mono valve doesn't look all that old.

I never had trouble with heat to the cabin itself, just getting it to the floorboards.

Does the mono valve affect the interior distribution?

Anyway, it looks ok. I really hope it doesn't come back to the CCU replacement.

Thanks again. I'll entertain any other theories out there, too.

Gil