99 LT model.
I have replaced some bad vaccume lines I found
that were bad. The system works on all the settings,
floor defrost etc. A/C blows cold, etc.
I am not convinced that I don't have a vaccume leak
somewhere. Doesn't seem like the flapper door that
moves to heat is working.
I was wondering if anyone has come across this problem in
the past and if there was a cure. Thanks in advance, Al.

 

Check out the links below.
GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

 

Are you sure the lines running to the heater core itself are hot and circulating?
Cores can get plugged up on these trucks.

 

I had not heard that.
I will be checking in the morning. Thank you.

 

Great link. Thank you very much.

 

So here's the scoop on this one.
I followed the directions from the link posted rockp2. Everthing seemed to be working as it should.
OK, what's next... Warthogdriver says it could be stopped up, hmm...
Removed the lines to the heater core and the large 3/4 line seemed dry. DO NOT use compressed air, lol. If you do, you will be blind for a few moments hunting fresh water.
Took the water hose and fushed one to the other, back and forth 6 or 8 times and replaced the lines. It worked! I didn't see a bunch of crud or calcium deposits come out, but I think there might have been an air bubble causing it to vapor lock possibly? It is blowing hot now.
Thanks for all the great advice guys, Al.

 

Thanks for posting your fix for the rest of us that may come upon this problem.

 

This is usually the first thing to do when you are not getting heat.

 

I figure if you use the forum to try to find
answers to problems, it should be common curtousy
to let folks know how you got things fixed.