Originally Posted by xlt4wd90

The top of the canister has a vent hole that is supposed to open up when the engine side gets vacuum. There is a valve in the line going to the intake manifold that does this, when the EEC decides to. But supposedly the vent is only supposed to open when the valve opens up to allow drawing of the fumes into the intake. It's not supposed to allow fumes to leak into the atmosphere, which would defeat its purpose.

There may be more to it, but this is about all I remember.