GAS Cap,Pressure question

[gary h]

Stopped for gas today (Filled it) after driving 50 miles and heard/felt a swoosh of pressure when I removed the gas cap. Not sure if it was a vacuum or burst of air .Car is a '71 GTX Stock, with the vapor tank under the 1/4 panel (all lines connected).Drove another 3 miles home and noticed a small puddle of gas on the ground after parking. The car ran fine and outside temp was in the 90's.
So do I need a new gas cap or what ?

 

[WileE]

Vapor line may be clogged.

 

[ddpsnake]

do you have a vapor line going to the breather cap on the right side valve cover,you should not have pressure build up like that tho.

 

[YY1]

If the vapor manifold is anything like a 73, there is a brass shuttle valve inside one of the tubes )outlet IIRC) that can become stuck.

Banging it my unstick it, but it may get stuck again.

I drilled mine out. Was a royal PITA but I won't have that problem again.