1978 a/c controller troubles,

[duck652]

So basically - I hear this little leak coming from right behind the a/c controller. The fan speed and temperatures both work fine. The air always blows through defrost, though. The selector lever moves fine. The leak comes from this light green wheel with a hole in the center. Covering the hole with my finger does however allow the air to move to the other vents occasionally. My guess is a missing pin that goes on the green wheel and through the hole.
Any help? Thank you in advance!

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[ak7an]

Vacuum leak.. The HVAC system uses vacuum to change where the air blows out. Since you hear it right behind the controls shouldn't be hard to locate.
Take care
Ed