Fuel Cap on Stock Fuel Tank ('69 Camaro)

[ROKN69]

I have a stock fuel tank with a "vented" gas cap. When I drive the car around, the gas tank gets what I would call overpressurized. Especially when I'm racing. When I take off the gas cap, there is usually a LOT or pressure released (gas vapor). Is this normal?

I've tried two gas caps (same vendor) and both are supposedly vented, but it doesn't appear to be venting... unless it is supposed to only vent one way? If this is abnormal behavior, does anyone have a recommendation for a good gas cap?

I've asked at a couple of car shops and no one seems to know what I should do.

Sorry to post such a dumb question, but I don't know where else to turn.

--Roger