72 C20 in-cab fuel tank head scratcher (non-EEC, vented cap)

[jocko]

Bought this tank from Classic Parts for the 72 C20 and the ad stated that “thou shalt” use this gas cap (for a 71 non-EEC).

From the ad: Gas Tank- w/o EEC-New for 1968-72 Chevy Truck:
***Note: Does not work on 70-72 with EEC*** Includes sending unit gasket and lock ring. Must use our number 23-108 gas cap.

So.. I bought that cap to go with it, ugly warning sticker on it and all.
The ad states the cap is vented, which is what I need with this 72 C20 non-EEC setup (and I was under the impression ALL C20/30 were non-EEC tanks?? - regardless, the one I removed was identical to the one I installed). Tank plumbed correctly with return line, etc.

All is installed and truck is running again – BUT.. it does not appear to be a vented cap – when I remove the cap after the truck has been running for a bit, you can hear the tank hiss loudly and release the vacuum and “de-imploding.” So, I HAVE a vented cap (which may be defective, gonna try another) – not sure what else my issue might be.
Any thoughts?