Recirculation mode or Max, will close the cowl vent and the kick panel door. There's a vacuum diaphragm under the cowl door, that's what the bulge is for. Unbolt the door surround and remove, suck on the tube and see what happens. May be all rusty and junk or a cracked hose. Check the condition of the cowl block-off seal while there, can now buy them new, although just a piece of curved sponge rubber.
78 AC Vacuum heater diagram by
Ben, on Flickr
Firebird cowl air recirculation vent door vacuum setup by
Ben, on Flickr