69 350 2bbl no AC. Should be vented gas cap, i'm pretty sure? Cap says vented if that means anything. After a few minutes of driving if I open the cap there is a whoosh like it was sealed up and built up pressure. It sucks in. Is it safe to assume that my vented cap is no longer venting both ways properly? The venting valve in the cap moves freely. But the default position appears to be closed, so preventing blowing out as a default. My problem appears to be sucking in. Cap won't help with that? Also I don't see any vent tube near the filler. I have really good records for this car and I don't see that the tank was ever replaced.