I have a Meha where the first valve piston is shot. My tech says it's smaller than the other three, that's why it sticks on the way up - someone must have replaced it or honed it down too much.
Since it's Monel, adding more material by plating would be hit-or-miss, it might come unstuck eventually, he says.
Does anyone have a junker lying around? Maybe one where the red rot has eaten away everything but the pistons...
Kanstul, being in the process of closing shop, were not able to help me. They just have a sales guy and a lady doing the books, apart from Jack. They're selling off surplus mouthpieces at the moment, looks like.