yes, you can have a biased (hooped) evap pressure sensor. but before you condemn a sensor you need to make sure it's reading things incorrectly.
the purge valve (under hood, and not to be confused with the vent valve @ the canister) is normally closed. i have done many a ford purge valve for sticking open. the valve (solenoid) will work electrically, but mechanically is fails to function.
easy test: disconnect both lines on the purge valve - one goes to the canister and the other goes to the fresh air intake. blow into the purge valve from the canister side - your breath had better not come out the 'outlet' side.
another common complaint from this failure is a hard start/long crank after fueling.
2003 GMC Sierra 3500 -- dually, 4x4, hasn't blown up yet and that's what i look for in a truck
Ford Doctor -- They come in broken and leave fixed. Ford of Canada Master Tech