Not many things can burn up brushes that quick.

When you get the alternator off, pull the brushes and have a look at the slip rings. If they are pitted and rough, that would make the brushes wear faster. HOwever, that sort of thing should not be there in a properly rebuilt unit.

If you get one at a boneyard, do the same thing. Also, spin while the brushes are out. You are correct in checking the bearings that way.

Minor pitting can be cleaned up. I use a 400 grit emery paper, cut into a strip and held over a fingertip bearing on the slip ring while I spin the rotor. Didn't take long, as the slip rings are brass.

Good Luck,

Bob

:>)

If you go with a larger amp capacity, put in a fatter ground wire, too.