When should brushes be replaced?

They just need to be replaced before they've worn completely through, to prevent the spring from contacting the commutator, which would quickly damage it. Worn brushes shouldn't affect performance, though it is a good idea to clean the commutator and brushes while into a motor - I normally use Q-tips with isopropyl alcohol.

Recently I've been reviving some early Marx locos. I replaced the original brushes because I didn't want to polish them smooth, even though they had plenty of life left.

Along with 91% isopropyl alcohol, I also use Deoxit (Amazon) on the brush holders and commutator with good success. They run much better now..

I replace them when they need it.  If they get to be about 1/2 the original length of a new brush, I figure it's time to stick a new set in.  One thing about short brushes is the tension on the commutator starts to get lower, which will affect running.

@MR_P posted:

I replaced the original brushes because I didn't want to polish them smooth, even though they had plenty of life left.

Personally, I'm not convinced replacing brushes improves anything. Both the new or old brushes will expose new material to the commutator after a period of running, so the end result is the same.

@Johnbeere posted:

Personally, I'm not convinced replacing brushes improves anything. Both the new or old brushes will expose new material to the commutator after a period of running, so the end result is the same.

I should explain that while replacing the brushes for a new set I also clean the commutator...

"Along with 91% isopropyl alcohol, I also use Deoxit (Amazon) on the brush holders and commutator with good success."

I suppose you could just clean the commutator and the brush holders but brushes are about a buck a piece and I don't want to replace them again, even for a 1930's engine.

That's fair enough, I'm just wondering if someone can provide any reason that new brushes would perform better apart from what's already been discussed (that the new ones will be cleaner, and to gain back the spring tension).

I do it to gain back some spring tension.  The springs only have so much travel, then the brushes are floating in the holder and aren't pressed against the commutator.  That happens sometime after they reach half-length, which is why I replace them at that time.

Because old brushes can infuse moisture and oil, the already mentioned tension issue, and the mentioned carbon deposits on commutator.  A brush change normally includes proper cleaning.   It definitely can effect the old pulmore motors and how well they perform.  Same to a degree with old PW pick ups.  Running AC motors and once pickups attract moisture notice how some engines spark constantly at the pickup with rapid wear.  I have a small MPC alco in that phase right now.

Can happen to MTH pickups also, I have seen plenty of pickups that have an ok appearance other than a darker ring around contact with rail, but completely erratic at providing good middle rail pickup.  Changing pickups and issue is resolved.  G

Thanks for the answer, I didn't know that could happen.