How exactly is this vent supposed to work? Every time I remove my fuel cap I get a big whoosh of air. Can the vent be replaced alone or do you have to buy a complete top assembly for the tank. Or is this normal?

 

Both the vent on top of the tank and the fuel cap are made to hold up to 3psi pressure or vacuum. A hiss is normal. What gets people in trouble is using an aftermarket locking fuel cap made for gassers that doesn't vent. In combination with a tank vent that is clogged or broken I've seen this to cause the tank to collapse.
The tank vent is available separately for under $25 but you may have a hard time convincing the dealer of this, most are too lazy to take the time to look it up and would rather sell you the entire $300 module.

 

Thanks for the info