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I have a Willson 2900 that works great, but one thing is bugging me with it. While the valves seem to work perfectly, I have a problem with one of them. It is really tough to pull it out or put it back in when I want to clean or oil it. After it gets set back in the casing, it works great. I try to be real gentle with it, but this can't be natural. It started doing this about a month ago, and I don't know what would cause it. I take great care of my horn. What should I do?

Nick