Use grease on alloy caps to prevent a chemical reaction.

Smearing the brass valve with Vaseline or grease before replacing the alloy cap should resolve the chemical reaction problem.

Asked on 2 February 2013 by AO, via email

Answered by Honest John
I have found that, contrary to the advise of aeronautical engineers, Copperease grease smeared between alloy wheels and iron hubs prevents them from fusing together. I don't know about these things, but I guess the introduction of a third metal affects the anodic reaction.
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