Apparently you have more than a brush problem - the slow motor indicates the brushes are not making good contact.
When that happened to me several years ago, I found that all the plastic parts at the top of the motor had melted (the insulator and brush holder - not available any more.) due to the heat generated by the poorly contacting brushes.
When I e-mailed Sears asking for a better picture of the motor he suggested trying 257.797901 and THAT picture was clear.
Suggest that you access sears.com and try that number.
Be sure to order TWO brushes and TWO tension springs.
The answer to your question - look at the diagram for the model number above.
Take the motor housing apart (7 screws), remove the wires to the brushes, remove the tension spring (not easy), then remove the brushes.
Reverse to assemble.
I am now in the process of changing the brushes in my (exactly same) model.
anon, April 2007
