my sentiments exactly, sounds like a vent valve is not opening or closing, the "hisss" sound when you open the cap is normal when the tank is almost empty, but it should not be so bad that the tank itself actually changes shape due to the pressure change. it does sound like the pressure in the tank is so high that its pushing fuel up the line past the fuel pump even when its not running. there is definatly a problem with your evap system somewhere. i suggest you stop driving it on the road, if your fuel tank is deforming that much under pressure its not safe to have on the road.
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