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I was approached by a guy that works at a place I teach at about issues he's having with his Euphonium. The horn is a York Eminence 4052 that was produced just before the factory closed. The horn's valves are reportedly "banana" shaped. I don't know what thisbmeans, but he has had them looked at and lapped on three different occasions at various shops and no one can seem to mend the problem. Any ideas? He's willing to get them fixed or search out new valves to get this horn working.

If the pistons are bent, lapping is only making things worse. There is a good chance the valves will need to be rebuilt. Pistons and cylinders should be measured (with precision to 0.0001") and compared to see where the damage is.
-Kevin

That was my thought. I haven't actually seen the horn or valves, but figured that the bent piston option would surely have been checked by a tech one of the three time he brought it to a tech before they recommended lapping. I'll see if I can get my hands on it and take a look at the pistons.

I guess my real question is whether or not this is common problem with the model or if this is just a dud.

I've never had valves rebuilt, so who should I send this guy to for a quality job? Who do you guys know that would be, or who here is, good at and willing to take on such jobs? Thanks.