Was going to call them 'gas caps' but that doesn't apply here.

Should they be vented? I know when I've opened them up the last couple times I've had quite a bit of air come out. I also know my grandparents crushed one of their (Ford, gasoline) fuel tanks using a non-vented cap, so I thought I would ask.

my truck blows some air when i pull the cap off its sposed to hold back 5 PSI presure then let it go