1970 fuel cap ??

[fotyfobravo]

Original 1970 1500 GMC truck. I have bought 2 replacement fuel caps. BOTH seem to be vented. But .... After about 10 minutes the engine stalls out and I remove the fuel cap and....wooosh....the air rushes into the filler and then the truck will crank again. If the cap doesn't vent, a vacuum is created in the fuel tank as the fuel is pumped to the engine.

Is there another vent other than the cap that might be plugged up? I have looked under the cab and only see ONE fuel line leaving the tank and running forward to the fuel pump.

Thanks y'all....