If you have a US spec fuel vapor evaporation system in place still, this is an example of after-engineering to meet US Federal nit-picking.
It weighs five pounds and includes twenty feet of 1/2 inch SAE fuel line to accomplish this, as well as the nefarious "one-way valve." It is "open to atmosphere", but only on the far side of all this plumbing, TWO carbon canisters, and vacuum applied from the intake manifolds.
I'm with Kiwi_Roy (always a safe bet! ) that something got pinched. *Might* be a good time for the Tank Off Maintenance Checklist with a good mind toward the venting/ overflow plumbing.