A/C changes vents that it comes out of by itself

A couple of times now the air flow with the air conditioner on has changed from blowing out of the vents in the front panel to once blowing on the floor and today blowing out of the defrost vents. It only stays this way for a few seconds and then you can hear a door somewhere change and then it comes back to blowing out where it is supossed to be. Do I have a vaccum leak or is there something electrical that can cause this? I am not sure if it does this only with the A/C on or what but it just started doing this and its too hot to drive around with the heater on and see if it does it then too

I had a 95 C/V that did that whenever going up hill. It was loosing vaccum. There is a vaccum resivour, a very small black canister at the firewall. I removed mine and made a large resivour with a soup can. problem gone.

I forgot to mention that this is a 92 C/V 3.3L if that makes any difference

There is also a check valve in the main vacuum line that goes to the heater controls, if that is bad the air will switch vents when you are accellerating when the engine vacuum is low.

I will check the canister, where is the check valve located? Is it in the engine compartment or under the dash?

Thanks for the replies.

I have noticed that when it does the vent change if I take my foot off of the gas pedal it instantly goes back to when it should be.

The check valve is usually under the hood, there is small line that comes off of the power brake booster, that is the line that goes to the heater controls, somewhere in that line would be the check valve.

I dont have a vaccum operated brake booster, its electric, but I did trace the vaccum lines to the plenium to a vaccum nipple. where there is what I am assuming is a check valve inline. the vaccum line was connected but was loose and not getting a good seal. I am hoping that this was the problem.

Thanks again to everyone who responded

The loose connection that I found didnt fix the problem, and the one way vaccum valve is good.

The vaccum line goes from the plenium straight through the firewall and there is a return line that goes to a shutoff valve for the heater hose. I am going to have to assume that the vaccum canister is beneath the dash on the inside. Does anyone know how hard it is to get under there and trouble shoot the duct door system? I looked under the dash and all I saw was the plastic of the ducts themselves, no controls of any sort.

Any suggestions ?

THanks again!