GMC Repair: heater/ac vent problem, gmc jimmy, air brakes

Question
I have a 1994 gmc jimmy.
just replaced blower switch.
but am having problems with vents changing positions. from defrost, to vent to floor.
right now air comes out of all areas at once.
What must i look for to repair or fix.?
I used to hear the changing of the doors so to speak. now all i hear is a sound like air brakes releasing when changing.

Answer
hello eileen
this is worked by vacuim in the heater controll i would pull it out of the dash and thire is one with a hole in line or came off in a shop to do this job would be 100 with labor good luck to u hun and you have a pretty name and if you need more help just let me know pete