I know a LOT of people who usually just stick to this forum for their vintage vehicles (myself included). As for your problem, I can't address it as my 108's lock is on the door itself and not the cap. I do know the cap can be sticky sometimes, however. I put a new seal on it a while back and while it seals well, it does require more effort to remove. Not pickle-jar effort, but more than my 2 newer vehicles with "Clickety" caps.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k