[f5fstop]

Yes, there is a vent valve in the system to allow air to flow through the carbon canister when the engine vacuum is sucking the vapors. If this valve is stuck close and the EVAP solenoid opens to allow vacuum to suck vapors from the tank, the tank will eventually start to crush in on itself.

However, the system will run a self-test to verify that the vent valve is working ok.

I would check the canister itself to see if it is plugged, or it might be the anti-siphon valve at the end of the fuel fill neck where it enters the tank. (Anti-siphon valve is used to prevent people from siphoning fuel out of the tank.)