what is this thing?

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The selector switch on your dash that determines where the air comes out of (vents, defrost, floor) is run by that vacuum canister. If you remove it, you wont be able to change where the air comes out of... you will have a dead switch. But, you can remove the ball cannister and then run the small line that comes out of the firewall near the heater core pipes to a vacuum source and still have an operative switch. It just won't work unles the car is running though.