Thanks to a forum member for sending me fuel system diagrams I figured the "vent valve" I had put on under the tank was for the engine bay, I had no idea there was two of them and I didn't have an example of the correct one under the tank cause mine was missing...

I'm guessing it just built up pressure until it collapsed, I'm going today for another tank and the fun task of pulling and replacing... Even with the correct vent valve under the tank I'm a little hesitant to replace it in fear of it doing this again... I just want to be done with my fuel system...I hate gasoline.

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76 MG Midget
78 Datsun 280Z
86 Peugeot 505
01 Subaru Outback

“Owning a car is like owning a dog. Every now and then, it craps on the carpet — but it’s still your dog.” - Jeremy Clarkson