I also have the AC servo unit and have had trouble with it for quite a while.
Looking forward to forking over the money for a new one just to get the
whole package working like it is supposed to.
But... when it got cold I bypassed the unit and ran hot water straight
into the cabin (just like the other posts talk about).
I seem to remember having to replace the small electric auxillary pump too.
A salvage yard had the aux pump for $25, not too bad.

I tore apart that AC servo looking to fix anything I could find.
Darn confusing. Did have a broken arm that I tried to JB weld.
It did not work. Hence the search for a new unit.

Anyone ever replaced the AC servo in a 1980 with some other year
method ? I have a 1981 donor car, but just can't see what needs to be
switched to eliminate the 1980 AC servo unit.