I replaced the old loose cap on my 66 C-10 & I cannot believe the amount of pressure that the tank builds. It will actually try & blow out of your hand as you twist it loose. It is a vented cap but I guess I dont know what they mean by vented..vented to be means there could be no pressure or vacuum built. I took the cap off & I cannot blow through the valve but I can suck through it. I tried to change my fuel filter all the way up by the carb & when I released the clamp it was like putting you thumb over a garden hose. Is this just how they are or am I over thinking this?