Ok, folks...it isn't so hot yet this morning, so I got up into the attic and found the original AD gas cap. The one which GM put on my 1952 when it was new. Please see below 2 images and we can see how this was designed. Image 1 shows the vent provision in the AD gas cap. If I suck and blow on this, air moves nicely in both directions. Feels about like the amount of air I could displace blowing / sucking through a tube of maybe 1/8" to 3/16" ID...not any more than that, however. And if you look at image 2, you will see there's a space between the cap and the brass piece which (along with the rubber ring which fell out when I took this out of the envelope it was resting in) forms the seal for the filler neck. Air escapes in/out from beneath this brass piece. Neat design and hopefully this question is solved.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
