It's been a long time sense I bothered to open up a starter. First one I had I opened it up and cleaned it good with electrical terminal cleaner and then new brushes. It added some more time to it's life. Then when it went out again I found a new one in the box at a club swap meet for 10 beans. When it slowed down I just put it on a shelf and went to Hobby People while they were having a sale and picked up a Hanger 9 Power Pro HD for 20 bucks, it's still with me today and using one of the HF 18 volts John mentioned. It will roll over a DLE 30 on a fresh charge.{Usually!} I tried the 18 volt pack on the old starter on the shelf and it got with the program too. I shot it good with the electrical cleaner, it's now a back up starter.
My point to this ramble is sometimes it's just better to cut your losses and buy a new one. Just something to think about.