Fuel tank pressure??

[akart]

I've tried to look up fuel tank pressure,not seeing a good answer. Looking @ vented caps (GM) and they say on the cap "Warning Remove Slowly Fuel spray may cause injury" What's with that, is pressure normal? Don't have a canister. have a 2 tank system but only one works,right side.Using new Stat locking cap listed for my truck.I think the frame and tank system is a 1974 K20. I'm going to drill a 1/16" hole in the cap. Any problem w/that? That pressure has to be screwing something up. Edelbrock carb and pump.Edelbrock carbs don't like high pressure. Is my fuel pressure to the carb effected??