The tank is supposed to do that. When the system is stock it pressurizes the tank as the fuel evaporates. That EVAP system only allows pressure out when the system is running as it is solenoid controlled. Solenoid only opens at a certain PSI to relieve the pressure. When it sits overnight, the tank is low and it gets hot. It has more room for the fuel to evaporate and expand. I believe the pressure control helps to stop fuel from evaporating right out of the tank.

When the Evap system is removed the tank should no longer pressurize as you are venting it to atmosphere and there is nothing to hold in the pressure unless you have crimped your vent line in which you would have other problems like not being able to fill your tank without it spraying back at you.....LOL Ask me how I know!!!