46 super gas cap

engr1962

Took my 46 super for her first drive up to the corner gas station for her first fill up. Got there in one piece, but when I opened the cap, it either needed to breathe in or out, couldn't really tell, but there was definitely a pressure change when I took off the cap.

I seem to read that it should be a vented cap. How can I tell if it actually is vented.