Rear gas tank vent question.
 

1970 c10
350/350

I installed a rear aluminum tanks from performance online this week. Nice tank baffles, etc. can't even see if it from behind
So I purchased the roll over vent for the top of the tank. Put some 5/16" rubber hose on it and put a pig tail in it per tech support.
When I take off the filler cap i hear swoosh of air.
It's my understanding that the roll over valve is and open vent lettin air in and out.
Any ideas?