Interesting idea with the dry-erase markers. I may try that if I end up having this bike hang around for a while.
I'm surprised the leather washers should be so difficult to get. I know there are companies that sell them for other purposes, and those might be modifiable to fit the pump. Otherwise, there are definitely people around here doing custom leather work, and one of them should be able to fabricate something with the right dimensions.
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If someone can pour a Guinness with a cycle instead of a shamrock on top, I'll update my profile pic.