help with air condition 85 cutlass

[beatupdablok]

my air blows through the defrost and floor vent but not through the driver vents or passenger vent...??

 

[hc243102]

Same thing with my 85 cs. Only blows from defrost/floor. Then after about 10 minutes theres a loud bang that scares the hell out of me every time and viola it works as it should. I have a vacuum leak thats coming from behind the ac control panel. I think ut might be the selector switch. Can you hear any hissing?