IIRC, the distribution function is handled by a lever from 1973-76 models. The vacuum line from the engine goes through the firewall into the function lever switch. On the pushbutton controlled A/C, it was slangly called the harmonica. If everything else works well, suspect the hose from the harmonica to the dashpot that controls the defroster door. Check to make sure the harmonica isn't cracked, too. You can test the dashpot, too, by running a suction hose directly from the engine to the dashpot. If it moves and does not hiss when you connect the hose, it is probably OK.
