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

Same thing happens every time with mine. I have a Chinese one from before 1990 and a US made one bought about 2009. The same issue can happen to a Fuel Injection Pump that has sat on the shelf for years or even one on a Car that has sat for years.

Ones still on the car with all of the connections intact seem to do much better because there is no place for the fuel to evaporate because it is sealed off. However, sometimes rust forms inside the Fuel Injection pump due to the water content in the fuel.

Note WD-40 or some other penetrating oil after the delivery valve is out works to free them or at least help. If not an aluminum or brass punch gently tapped on the top of the plunger usually frees it.

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84 300D, 82 Volvo 244Gl Diesel