Did you take it all apart?
If there's play in the bearings, replace them, they're cheap.
If there's a lot of wear on the armature where the brushes rub, you should smooth them out. The ideal way is to take a light skim off them in a lathe. Sandpaper works too.
Check the windings for continuity. Both the field and armature. I'm sure the book shows where to test.
My alternator has over 250k miles on it!