stock alternator burns brushes

I had one do that. It went to 20v.
Bear in mind that running wide open with zero regulation,(that's full-fielded) that alternator can practically be used to weld with. I have seen 60 Vac come out of one. So, if the regulator cannot tame the beast to 15ish, thru the ground circuit, then it's defective.
I replaced mine with one from the junkpile and away we went.Oh yeah that's a funny story by itself. I had about a half dozen spares.And they were all regulating at different voltages; from about hi 13s to hi 15s. I picked one that hit 15.2 and called it good; that was a good 10 years ago. My alt is a cobbled together item with each major player from a different alt.( I had a box of maybe 15 of them). Over the years I have replaced each player (coil,rotor and diode pack) at different times at least once. And the brushes several times; once even with new ones,lol. I tell you tho, replacing individual diodes was my least favorite operation. But I figured it was good for my education. After that, I just reconditioned a couple of alternators for spares.I carry spare brushes in my spare parts kit, with a ballast resistor and bulbs, fuses, etc.