I assume this is caused by the vacuum storage canister off the brake booster?
It looks like a 35mm film canister like others have said. If you blow thru this it should only blow air one way?
Mine acts like it's hollow. When you accelerate the vents change direction.
I'm not familiar with this item. The dealer has the P/N 4677204 but it has 3/16 lines not 1/4". I think mine has 1/4 lines off bottom. I guess I will go to the local u-pull it and try some off other cars. Any ideas would help, thanks!

 

When you say change direction do you mean it switches from defrost to floor to vents? The vacuum should run off a tap on the intake, inside under the dash there is a small pancake shaped diaphragm, the line on one side is larger than the other. To test this you are going to need a vac. tester, its simply a hand held gun with a vac gauge, hook this up to the diaphragm the side that goes to the firewall and watch the gauge if the diaphragm works it shouldnt bleed down, if it does you might have a bad diaphragm.
Does it act this way at idle?

 

Went to local upull and got a vacuum canister.
That seemed to do the trick.
Thanks for the replies.