I just went through this on my 72 challenger. Take the cap off and take a small piece of hose and put it on the the hole you see in the center of the inside of the cap and try to blow through it I bet you can't. Don't try to blow through it with compressed air like I did because it seems like it works with 100 psi but if you had 100 psi in your tank you would probably have a sending unit sticking in your back after the explosion Is it a Stant cap? I think you got one from the same production run that I have. The vent plunger is not drilled all the way through. I took a small drill bit and finished drilling the plunger in the center of the inside if the cap it works fine now