"The one MB I've owned puts out about 13.25-13.30 at idle with engine fully warmed up. It reads higher after initial startup when cold. It read a bit higher a few yrs. ago. I suspect a set of brushes might kick it back up. One of these days I'll deal with that."
I doubt if that is the culprit. Unlike with DC generators, the brushes in an alternator carry very little current. Also, they are actually slip-ring contacts, not commutator brushes, so there is no interruption of contact to cause sparking. They allow the regulator to adjust a magnetic field in the rotating iron structure - a spinning electromagnet - which then induces an AC field in the high-current fixed windings.
Although I have not experienced brush failure in an alternator, I would expect it to cause an erratic, then complete failure of output within a short period of time. I HAVE found many instances of diode failure, disabling one or more of the three high-current fixed windings, and this causes a reduced current capability under load, with resulting reduced voltage. Most other faults cause complete failure.