Is it worth on a trumpet I got for a steal at 300?During normal playing usage, valve casings wear in a "bottle of coke" or egg-timer shape, large at the ends and small in the middle. They also wear oval, with the large dimension at right angles to the lead pipe. This wear causes leakage, which makes the trumpet hard to play, or more "resistant"; and chattering of valves, which induces them to stick. To rectify this, the casings must be precision honed to make them perfectly cylindrical. Then the valves need to be trued cylindrical and then plated to bring them up to an appropriate diameter for the casings.
It is very common for an instrument requiring this amount of treatment to need other repairs as well, however the valve rebuild, or piston refit part normally runs $400-450.