VB wrote: <Third, maybe fourth hand info (from a french horn player, no less!), but one famous repair tech said that at the customer's request he fixed the compression on some vintage old french horns (slides and valves) and they played much worse than before. He speculated that they were sort of designed with poor compression in mind and would still play fine.>.
But is not the "heavy oil test" the answer? I have an old London Besson with worn and leaky valves, with a bad low register. I oiled it with arachid oil and the sound was great. So why should a valve replating not achieve the same result? To sum up, in case of doubt, I would make the oil test and if the result is good, have the valve rebuilt.