A/C not working in 2002 Chevy Blazer

My A/C in my 2002 Chevy Blazer
quit blowing cold air.  When
checked with a guage it showed
overfull so drained all freon out
but when tried to refill with new it
keeps red-lining on the pressure
guage after only 4-6oz (calls for
roughly 26oz).....What do I do?  I
was told that an automatic door
that switches the airflow from hot
heat to cold a/c could be jammed
but I'm unsure where to even find
that to check it?  The Dan and
compressor both are working so
can anyone point me in the right
direction to troubleshoot and fix
my a/c please?  Thank you......