2000 Chevrolet Silverado 1500, air only blows out of defrost vents and not front air conditioning vents

SOURCE: Heat wont blow through heater or defroster vents

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

SOURCE: heater control stuck blowinf hot air out vents

did you cheked if there is enough refrigrant ( freon ) in your tubes by measuring the pressure . if there is you may need to check your condensor its may be clogged and not letting enough air comes through to cool the refrigran

SOURCE: All Air Conditioning blows our the defrost vents

Some thing you could look at would be this. The control is usually fed by a vacuum line or small hose thats is connected to your intake manifold.
With a cold engine,look for a skinny vacuum hose that runs from the intake toward the fire wall . It should be going inside the cab and connecting to the A/C vent control.Some times the hose becomes disconnected or splits or cracks due to heat under the hood.
Thats usually the trouble.
I hope this helps you.

SOURCE: 1978 Chevrolet Corvette Air Condition Problem:

I had this same problem with my 78. If your have vacuum going to the control panel and the control panel is plugged in correctly to the vacuum hoses, it is either a hose problem or the actuator motor used to open that particular door is no good.
I would get some vacumm tubing and replace that first since it is cheapest. You will have to take apart the dash by removing the radio, the guage cluster and a few other parts. Make sure not to confuse the hoses as each vacuum motor must coincide with a numbered "port" on the control panel. This is a pretty big job, so don't start if you don't have a full day to jack with it. If you replace the hose and it still won't work and the control assembly is hooked up to the correct hoses, try changing the actuator motor. If you need to know which hose goes where on the operating assembly, let me know and I'll put it up. It is nice once it works correctly. I had to replace everything though, including the contrrol assembly.

SOURCE: air conditioning not blowing thru the vents

There is a black vacuum hose in back of the glove box toward the top connected to a gray hose about 1/8 thick.You need to check to see if there is vacumm there.If yes then you need to take the bolts off to be able to tip the dash back enough to see all the vacuum hoses.Make sure they are all connected.It was alot of bolts but its is the best way.I had a hose off going to one of the acuator doors.Glued it back on and works great