I need to correct a mistake that I made here. I have the main cap gasket installed wrong. I figured this out today when something just wasn’t making sense when I tried to put the cap on my tank. The illustration doesn’t quite explain this but I pulled out a third cap I have and took it apart. While it was missing an inner seal, the main gasket was on correctly. Here is the cap I pulled apart
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Here you will notice on the inside of the seal is a lip/groove where something is to fit in there.
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Well the 2piece cups that are sandwiched together are supposed to ride inside that seal lip with the thin end of the seal squeezing against the bottom side of the cap and the thick end sealing against the filler neck.
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Here you can see it properly installed on the cap. Much better!
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sorry for the misdirection initially, but that’s how we learn sometimes.