What's the deal with my gas cap?

[71CHEVYSHORTBED402]

Got some gas in the beast and figured it was time for a new gas cap. 1/4 mile up the street to Napa, I end up with seems the same cap I had on my 71. Cool.

Drive 1/2 mile to the store to pick up the wife some flowers, you know, anniversary. Start the truck and it's running like crap. Sounds funny too. I don't see anything, but my old gas cap is in the garbage at Napa, so a 180 to save my newly appreciated MotoRad gas cap.

My old cap doesn't seem to help, so drive home and discover the cap from the vacuum port on the manifold was gone. Huh?

Forgot to mention: It blew a cork when I pulled the gas cap off. Gush of air and gas. Seems to me these caps would be vented both directions. My old one worked on the 71 no problem. I don't claim to know much about caps.


Wish I knew more about caps and EEC. This truck no longer has the canister. As far as I can tell it's set up like my 71 was. Pump has a return line, which I believe would otherwise be to the canister.