stchasking

There is a film of something on the valves. The brass bores probably have some corrosion products interfering with the sliding parts.

Wipe down the valves with denatured alcohol.
Get some lint free cheeze cloth and soak it in denatured alcohol and swab out the valve bores. Make sure don't leave any lint or fuzz in or on the valves. Lubricate your valves with your favorite valve oil.

The air passageways in the valves may have corrosion products inside. Clean them with denatured alcohol. (Use ethanol or methanol. Pure vodka or Everclear.)

Let me know how this works. I have to do this to my Yamaha/Shilke pic when I haven't played it for awhile.

EDIT: Good spring metal does not work harden. Spring material has to be nearly a perfectly elastic material. The endurance limit is never exceeded in a well engineered spring. I would think cornet valve springs are well engineered and have a high life expectancy. I have never had one break. I have had two garage door springs break so they are a case where low cost drives the design. Also the springs in my refridgerator that support the compressor broke. Again, economics of design and product cost.

Play that cornet all you want with vigor and pride.

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