Not Euphonium, but Alto Horn, however the problem probably could occur on any brass instrument with piston valves...
I have a BM Symphonic Alto Horn that I really enjoy playing. It has a beatiful tone, I can really make it sing when I try. Intonation is excellent...except for one note. That is the (written) D in the staff. It is awfully flat, so I need to play that particular note with 1-3 instead of 1. Everything else is very good in tune.
Today I cleaned the horn thoroughly. After putting it back together, I started playing and suddenly noticed the D was in tune when playing with only 1st valve! Of course, I was very happy and dreamt of playing all the difficult licks better without the pesky 1-3 fingering. But alas! As I played on, I noticed the D getting flatter and flatter, until it got to the point I had gotten used to. So now I am back to the D with 1-3.
This has me wondering though...how can it be that just one note is so awfully flat? And that after cleaning it is perfectly in tune for a while but deteriorates so quickly? And of course, does anyone have any suggestions how to solve this?
I do believe the valves are leaking air very slightly, but it seems weird that that would cause a problem with just one note. To me it would seem more logical to cause intonation problems over the whole scale or at least all notes with that particular valve, but it's just that one D. And if that's the problem, could replating the valves solve it? But then, would it still play as nice as it does now? Because if replating brings a risk of changing the sound, intonation or overall playability in a negative way, I am more than happy to keep using the 1-3 combination for just that one note.