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The definition of dielectric is non-conductor or insulator.
So, by definition, the answer is that you cannot use a brass (or bronze or any other metal) valve to take the place of a dielectric union. This is not to say that it has not been done.
I have seen it argued that, if you use teflon tape on the male threads, you are, in effect creating a non-conductive situation and the effects of electrolysis would be greatly reduced if not elimated. Unfortunately, this argument, IMHO, fails to hold water (or anything else) given the fact that the teflon tape gets ripped at the peak of every thread, eliminating the insulation or dielectric effect.
Hope this helps.