Tested the purge and vent solenoids working fine electrically with active tests on Maxi Sys Elite . and system holds positive and negative pressure , however when testing system with pressure, not vacuum with a  paint can smoke machine that low pressurizes the fuel tank pressure sensor up to 1.58 kilopascals, the canister vent valve is unable to overcome the low pressure and  open and vent this very low regulated air  pressure applied, but it can still open and vent when applying vacuum.

 I do not know if that is a normal for GM Canister vent valve to behave in that manner of not overcoming low air pressure applied from a smoke machine.

    Also with 2/3 of a tank of gas (not Full) and removing the gas cap with the low applied  pressure and fuel tank pressure sensor reading 1.58kpa , doing so does not relieve any of the tank pressure from out it's  filler tube. If I remove the hose of the smoke machine from the canister port , then the pressure vents from the canister and the fuel tank pressure sensor returns to zero from 1.58 kilo-pascals , and only after doing that can the de energized vent valve open back up from it's  stuck closed position. Any body care to chime in?  

  If the car was still here I would probably try to unseat the ball with a long coat hanger down the filler tube to see if that is the case, however I suspect that the filler  tube extends further down into the tank submerged below the fuel level and the air pressure was too low to be able to displace and force out the fuel below it and come up and out the filler tube until the air pressure vents out the filler tube.

  I'm more curious to know if it is normal or not for a good GM vent valve to remain in the stuck closed position after being de energized while the  system is pressurized from a smoke machine, because normally the purge valve applies engine vacuum and not pressure to system