neilevans said:
Try removing the valve and clean off any oil on them.
Get some brasso polish and pour a small amount (but enough for a thin film) on to the valve. Replace valve in cornet work valve up and down for a few minutes. The valve will feel heavy and maybe sluggish.
Remove valve and clean the polish off both the valve and barrel, soap and water is best. Oil clean valve and replace.
If your cornet is lacquered be very careful not to get brasso on it, or say bye bye to the lacquer !!
I've done this before on older instruments but I wouldn't recommend it on a brand new Sov!
Besson valves are notoriously pants these days. I think the best thing you can do is take it back to wherever you bought it from and ask their advice. It may just be a case of giving it time but if it's really a problem now, I would tend to go for lighter springs first (ie different make not Besson replacements), but these may cause your valves to become sluggish and sticky. Make sure you keep the old springs! If that doesn't work and they have an in-house repair shop they
may (with Besson's blessing) try to lap the valve out slightly.
Do not try doing anything like this yourself though - you'll find Besson less that helpful with any subsequant warranty claims if they get wind you've been messing about with the most delicate part of the cornet!
As a last resort you can always try to get it sent back to Besson to get it checked out - if it's one valve in particular it may be slightly bent.