Gas Tank Pressure

[sjarrett71]

My carb is off the engine getting rebuilt and the fuel line is obviously disconnected. I came home from work to a puddle of fuel under my truck and popped the hood. There was fuel pooled up on my intake and fuel coming out of the fuel line. I took my gas cap off to the normal sound of pressure escaping and the fuel stopped running out of the line. It is a new gas cap. My truck is a 72 with a 350 and the rochester carb. All of the fuel lines are new as are the vacuum lines for the EGR and the lines running to the charcoal canister. Do I need a vented gas cap or is this normal? I have unscrewed the gas cap so pressure won't build and hopefully I won't lose anymore gas until I get the carb installed. Also, is it ok for the system to build pressure like this when I reinstall the carb?