larryp,
A malfuntioned EVAP purge valve may cause a very slight increase in pressure, if any. But definitely not enough pressure to cause any damage to a fuel tank. The charcoal canister would probably be the first to go.
I'm not sure what happened to the fuel tank in your car because I haven't seen it for myself. My guess would be that your car had a defective fuel tank to begin with and it slowly cracked over the years or your car has too much chassis flex.....maybe even a crooked mechanic.
larryp and logic,
Why do you guys assume that there is too much pressure in your tanks? Is it from unscrewing the fuel cap and hearing a lot of vapor escape? But anyways. Maybe the EVAP canister is over-saturated and is no longer able to "catch" the fuel vapors.
There isn't much reason to be worried about fuel tank vapor pressure because there isn't much pressure anyways. If you are experiencing hurricane like vapor wind and fuel splashes then yeah, be worried. I'd be more concerned with high fuel pressure in the fuel delivery system.