My pop tester has been sitting around unused for the last 3-4 years and when I tried it today the plunger inside got stuck in the up position on the first pump. I couldn't find any instructions on how to disassemble it. Eventually figured it by freeze framing a YouTube video. It's interesting that the one way check valve is basically a modified delivery valves you would find in an M pump. All it needed was a good cleanup to remove the varnish and it's good as new.
https://imgur.com/a/0VLpDrH

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CENSORED due to not family friendly words