Just replaced 3 of the Wilcox-Crittenden bronze tapered valves after 28 years. There was no corrosion, just some wear of the tapered plug and bore preventing a good seal, in turn causing a slow drip from them. They still all turned OK.
There are still 6 of the WC tapered valves working fine. These are the less used ones so have been less subject to wear of the bore.
We'll see how the Groco replacements fare.
Bristol 31.1, San Francisco Bay