All excellent advice here from Art. Just thought I'd add that checking for excessive wear grooves in the slip rings can be accomplished by sticking your finger in when the regulator is removed. Even with new brushes that are now touching, if the grooves are deep they may soon have poor contact. Also, between side slop in the brush holder and worn main bushing/bearing allowing lateral movement, the brushes can wander up the edges of a groove, making poor contact, also rounding or even chipping the brush tip for poor contact. I too have filed the slip rings smooth with success to extend the life of an alternator, running the commutator in a drill.

I've also had starter/alternator shops do a rebuild with great success, although increased labour costs are making that a less economical option now. I've also taken suspect alternators into these shops and had them inspected and spun up on a test bench to see if they're producing the rated amps, all for no charge.

I didn't think the brushes could be replaced in the newer Bosch regulators, certainly not easily. I stopped doing that years ago when brushes could not be found locally and shipping costs made it more worthwhile to go for a new regulator, always Bosch. There are some really cheap jobber quality regulators out there that shops sometimes use. They work okay, but I don't trust them to perform or last like a Bosch.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now