Tam,

I got under my '78 and both cables were snapped off due to seizure.

I don't know how willing you are but I would try this:

o    Lye on the floor facing the back of the bus with your head near
     to the back wheel. (You DON'T have to jack anything up).

o    Slide yourself under the bus keeping yourself lined up with
     the long axis of the bus. You only need to be just under the sill.

o    You will see the drive shaft just above your head, and of course
     the bottom of the engine. You can see a big fat looking tube 3 inch
     diameter bending upwards just ahead of the drive shaft. This is the
     Heat exchanger. (Mail me if you want to know all about them!)

o    Follow the tube along towards the drive shaft. It curves upwards and
     has a lot of interesting mechanisms at the top of it. Where you are
     looking now, you should see the Heat exchanger flap control rod and
     cable.

o    The Cable goes to the cab along the chassis and through the under
     shield. What usually happens is the inner cable gets rusty and seizes
     in the outer shroud.

o    Try and move the inner cable in or out with a pair of pliers, spray
     some WD40 on the cable.

Other than that, new cables only cost about 10 pounds.

Hope this help.

Lee.