Fuel tank question

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My fuel tank is just about finished but im unsure if it needs a vent in it? I thought i heard someone say before that if you didnt have a vent to let air in that the tank would be under a massive vacuum.

So my question is does it need a vent with a one way valve in it or will the filler cap (aero style fitted to body) do the same job?

Im running bike carbs with bike fuel pump if that makes any difference.

Any comments or advice greatly appreciated as usual