ventilation mode question

["Jack Gagnon" <bullitt@cybertours.com>]

I have a 87 5kcsq with ventilation control problems.  The system almost
always put air out the defroster no matter what mode it is in.  Sometimes
after baking in the sun for a while with the windows shut the system will
function properly.

When it is good and hot inside the car the econ and auto mode will blow
through the center end rear vents, in the defrost mode it switches to
defrost just fine, and in the bi-level mode blows through the defrost and
foot level the way it is supposed to.  I understand the defrost is the
default mode if a vacuum hose is disconnected, but everything seems to be
fine.

How does one explain the proper operation when the car is hot?

Anyone experience this problem before?

Any ideas?

Thanks

Jack