You can take your tank into your local shop, and they can install it for you, you just have to buy the kit (maybe 15.00, i'm not sure.) It's just a tube that is turned upwards in the tank, so that the gun gets air, instead of liquid, when it is cooler outside. I'll try to draw up something to make it more clear.
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Here for my very detailed, rather small digram of what it is.