Edward -
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When I turn off the heat into the cabin of my '81,  I still get heat
(but only in the summer, of course).  I checked the cable and the 'top hat'
and the little 'door" seems to be closing.  If I can't get it to close
correctly,  what if I disconnect the tube at the control in back
and let it vent the full diameter out.
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I, too, have this problem.  I checked into it last weekend while my 
dash was disassembled to replace speedo cable and install oil temp 
guage.

Here's what I found.  Although the door at the valve at the rear had 
room to travel, it wasn't closing fully.  I checked at the other end. 
On my '80 (probably same on '81) the left side control has a rigid 
and reinforced 90 degree bend at the end to go into the level arm.  
the right side has an end much like a safety pin, with a couple of 
windings that go around the bend on the left control and a hook to 
fasten back on itself.  This cable end is NOT stiffened, and not 
reinforced.  Mine was bending and not pushing the cable through the 
sleeve, thus not actuating the door.

My parts bus had a throttle spring attached between the rear of the 
metal and the nut on the clamp at the end of the cable, to assist in 
closing the door.  I tranferred them to my running bus.  Seemed to 
help in closing door without kinking the cable.

I was able to force lube fluid through the right side, but not the 
left.  I adjusted the best I could, and left it so that the travel on 
the right door is less that ideal, but still would close.

BTW, I used a motorcycle gizmo that clamps on both the sleeve and the 
cable and has a hole for a plastic nozzle extension for a spray can 
of cable lube that I also found at a motorcycle shop.

Disconnecting the tube at the back could lead to the vent system 
bringing in exhaust fumes, ack, cough, die, type of thing, so caution 
is in order here.

- Jeff