On those motors it is not uncommon for a brush to stick a little, which causes it to heat up, and the brush spring looses tension. Then the brush has even less pressure on the commutator, so it starts to arc, and pretty soon the brush has burned off - and the spring is totally toasted. The other brush can be totally OK.
That's something you'll never see come out of China. (Well, 400 pound people OR heavy duty treadmills. ;)
A loose connection on the back end can cause that. It usually burns up the wire's insulation. I had a brand new chop saw come to me with a loose brush. Ever since then, I've checked the brushes on all my new and used tools, just in case. (Cheap Chiwanese brand, money refunded because it ate the commutator.)
At a guess -- the square or rectangular tube that it slides in is a bit undersized or bent in a bit, so the tip of the brush was sort of skating on the high points of the commutator, drawing arcs whenever it should be descending to follow the surface.
Try moving the good brush to the other tube and see how smoothly it slides.
As a followup, I researched chop saws more after that and decided that an investment of $10 in a trio of Starrett hacksaw blades was a better idea. I go through about one a decade. Until then, I had no idea how much better a hacksaw could cut, or how long a good blade could last. I consider that a triple win for myself, all because of a cheap tool. So, even when unusable, cheap Chiwanese tools proved themselves quite worthy. ;)
If the copper braid pigtail develops a bad connection, then the thin spring has to carry the current, and it will get annealed in a second.
The braid is supposed to be buried deep into the carbon before it is pressed and sintered. Sometimes they cheat on material, and only a mm of braid is inside the carbon. Or, the braid could get fatigued, corroded or not be soldered well to the spring retainer/current contact piece. Any of these will fry the spring, and often the whole plastic brush holder assembly.
I did eventually buy a portable metalcutting bandsaw, a Harbor Freight portable. Noisy little bastids, but they work well. Bimetallic blades were the first (and yet, only) upgrade. I use cutting oil, for tough cuts of unknown metals, or Marvel Mystery Oil with it.
Has anyone developed a type of leakproof fluid container for use with portables, or is squirting the only solution? My buddy Glenn uses oil-soaked felt wiper strips on the blade of his 4x6.
A hinged mount and counterbalance will someday turn it into a fixed machine, but I'm too busy yet in my retirement to fab that up. This week, I'm painting my house. My masking machine fell off the top of the ladder and I had to repair it, too. I'm drilling new mount holes in the ripper extension today, to hold the blade firmly, but I got the new paper roll pivot built yesterday. Had to hack out washers to fit the bloody offset hole in the resin on the back. I may redo that by drilling out a pair, as it wants to wander a bit now. The longer I'm retired (first check hit my bank last Wednesday), the more I understand the gripes I've heard in the past which didn't make sense at the time. "Now that I'm retired, I don't have -time- to do that."
I have never seen a Sumo wrestler on a treadmill but they exercise an amazing amount. And the amount of beer they drink each day is astounding. Just part of their diet. If I worked out as much as they did with my present diet I would have zero fat left anywhere. Eric