I'm sure others know more, but here's my limited understanding of certain valve issues gleaned from my own well-used horns:
>Could be worn springs, which benefits from replacing the worn springs(You're addressing that).
>Could be some sort of buildup on the surface of the valve or inside of the valve casing. Best case it's just dirt that will benefit from cleaning... several cleanings. It can help to clean the whole horn too. Worst case it's some sort of corrosion (visible or invisible), which might improve with lapping: Taking some sort of abraisive compount and gently polishing down the high spot (Toothpaste < Brasso). The down side is that lapping is irreversible from a DIY standpoint, and can lead to...
>Could be excessive wear creating too large a gap between the valve surface and the inside of the valve casing. This allows condensation from your breath to build up in the gap. When this mixes with most valve oils it creates a viscous goop that causes valve performance to drop... sometimes a lot. I know of no DIY approach to this issue.
>Could be the valve got bent from being dropped or the stems got bent by a student trying to stuff their music folder into the case on top of the horn. Again, I know of no DIY approach to this issue.
The problem is that valves are touchy, and there are plenty of tech's who screw them up worse trying to fix them. So after making a few runs at simple self-diagnosis/repair, it's good to move on to a professional experienced with valve work.
Hope this helps; good luck; and most of all enjoy your comeback