I know this topic was taked about before, but could not find it. I have a 70 that builds up pressure in the fuel tank that it buclkles the trunk floor & the neck jambs against the bumper that I have to pry it up to get the cap off. It blows up like a bolloon.I have the original vented cap that I've inspected the check valve seems to be ok. I've tried a new GM replacement cap & a new vented NAPA cap to no difference. I was wondering if it has anything to do with the vent return line from the fuel pump. I have heard other guys with this problem but no answers. Anyone else have this problem? Thank"s Jim.

Jim,
If your 70 was built for use outside of California, there are no return lines from the pump. There is only a single line from the tank to the pump. The gas cap IS a vented type cap, which should allieviate the pressure. I experienced this problem during the 1973-'74 gas crisis. I purchased a vented locking gas cap from a Gulf gas dealer, but the pressure would build up to the point that the bottom of the tank would "bow out"! I switched back to my original cap and all was okay. I keep thinking that your cap is defective and that the new ones have a different check valve. I may have another original cap somewhere. I see that you're in Mountainside, not too far from me. E-mail me. You're welcome to the cap if it works.

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MalibuJerry350
TC Member #1279
Original owner '70 Chevelle.
580,000+ miles on car.
Hey, if it's got wheels, DRIVE IT!
My Chevelle: http://hometown.aol.com/erie614/myhomepage/index.html

Jerry,
My chevelle has a build date 6/70 from the Baltimore plant and shipped to Brooklyn N.Y.. It's a 396 SS uses the long fuel pump with return line to tank. I can't believe all these vented caps could be bad, but what you said about valves being different is possible. I was thinking of drilling a hole in one as my last resort, but I don't think that would be a good idea. I did the suck&blow check,and when I sucked in the valve was free and when blew out the valve locked up on both caps. does that sound right? If your cap acts differently, then I may be interested.I have one more cap to try ,but it'll be awhile before testing, I'm in the middle of replacing my windshield. Thanks for the reply&offer Jim