Plumbing question of the day: replacing a shutoff valve

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thanks lafinfil, excellent advice as always.

I used to make vacuum tight fittings using swagelock compression connectors and stainless steel lines in a previous life, when I was filling lasers with expensive and dangerous gases, and leaks were absolutely not allowed. We didn't wan to be exposed to those gases, and also didn't want air in the laser. Yes, I think I can handle this one.