No, the point of the system is to burn the evaporating fuel before it enters the atmosphere, thats why most states check the gas cap during the smog test. The pressure is from the fuel leaving the tank, the fuel returning (return line) and variatons in heat which changes the density of the fuel. You should almost allways have some pressure when you remove the cap, if you drive the car and remove the cap and don't have any then the cap is probably bad.
Depending on the config of your car e.g. egr etc. will determine the config of the evap system. My 87 N/A has no egr system so I run my canister directly to my throttle body because the bvsv valve on my thermostat housing broke.
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