a voltage regulator/rectifier on a bike without a battery regulates the amount of voltage put out by the stator so it doesn't fry other components, if the bike has a battery it converts ac power to dc power and recharges the battery, also regulating the amount of volts returning to the battery for charginng

 

No, the brushes (or alternator/generator for that matter) will not melt from the voltage produced by the alternator/generator itself.

The brushes simply receive/transfer whatever voltage is produced by the alternator/generator to the wiring and voltage regulator. This is regardless of the voltage produced--whether it be 1vt or 80vt.

The voltage regulator is downstream from the alternator/generator, and shunts excess voltage (amounts over about 14.5-15.5 volts) to ground so that your battery, ignition system, lights, etc, to do not get fried.

If the brushes or alternator/generator are melting, you have much bigger issues, such as fire or a short circuit.