My trumpet recently developed an issue. The third valve sticks at the same place every time I press it. With hard pushing, you can get it in, but then it's even harder to get out. I have taken apart the valve, spring, and surrounding slides and cleaned them thoroughly with every tactic imaginable. Everything works fine until I put the valve in, then it stops working. Any ideas on how to fix>

At this point, I would go online and buy a new third valve, if it's that bad. If it's just sluggish, you might be oiling it too often. That's what happened to my trumpet.

It could be a problem with the valve casing not the valve itself. Take it to a brass repairer - they deal with this sort of thing all the time.

I find Liquid Wrench works better than Trumpet oil except it smells a little;
for stuck valve, have you tried using a wet sand paper, try little at a time, avoid over doing it.

If you must experiment with 'sandpaper' only use a very fine grit - 600 or finer & wash off every trace of metal & grit before reassembling or the problem could become much worse. I still say take it to an expert before you ruin the casing as well.

Thank you! very useful information!