Clean out that 3-in-1 stuff and try a good high-viscosity valve oil. In addition to Hetman #3, I've used Roche "SlikStuf" in a very worn early 1920's Boosey tuba that's simply not worth the price of a valve job. Alisyn makes a good heavier oil, and I believe even Yamaha markets one, which might be easier to find in a local store than the other brands. It's still just a temporary small relief. But check for other worn parts while you're cleaning - valve guides, actuating rod ends, corks, any washers that may be present. Not that those things will fix your problem, but they can't hurt, either. Please don't read any of the threads here suggesting DIY honing by inserting the pistons in from the bottom. That guy made his own ill-conceived valve oil, too.