MaxA/C vs everything else...

I know that the MaxA/C setting recirculates air (reheating itself) but is there any other reason that I get heat using that setting and no heat from any other setting??? def/heat/vent/or other a/c settings....

'87 wagon, 4.0, auto, new hoses, flush/fill, new thermostat, new filler cap, etc.

Vaccuum problem??? I'm lost

Thanx

sounds like a old heater core,I had the same problem so i went to replace my heater core and man was that thing in bad shape,pull it and replace it should make the difference,or just clean it.but its not the easiest to do so i suggest spend the 39.00 and put in a new one.