Brushes are little bricks(or cylinders in the case of the Orion/Peak Revolution/Vantage motors) of a composite of copper, graphite & usually silver that has a copper or silver wire coming out of it & are usually soldered somewhere on the motor's endbell. The brushes are what complete the electrical circuit of the motor, as the battery's current flows through the wire(also called brush shunts) & the brush hoods through the brushes to the commutator(that copper cylinder on the motor's armature, the spinning part with the 3 segments that have wires wrapped around each part). And as the motor spins, the brushes rub against the commutator(also referred to as the comm) & both they & the comm wear, & as they wear, the comm's surface will start to become uneven, which will cause the brushes to literally bounce on the comm surface(which will cause a loss of performance & increase the wear of the comm & brushes), & it does make it neccessary to have the comm retrued(or having it cut, as many will say) after awhile, but how often really depends on the motor & the brush compound & type(like those revolution brushes, theycan definitely last a LOT longer than many more conventional brick-shaped brushes, & the amount of silver in them also affects how soon they wear & how fast the comm wears). But in your case, even with the Sprint compound brushes, I've heard of plenty of guys doing up to 20 runs before they feel it neccessary to replace the brushes & cut the comm......
(j/k)
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