The squeaking may actually be a vertical resonance bounce rather than a sliding friction issue. Quite a bit of research has been done on DC motor brushes over the years, including FEA modeling of noise issues.
Two issues come to mind: 1) The tension of the brush holder springs may be incorrect for the brush inertial mass and contact area. It is not uncommon for part of the current (amperage) to flow through the spring rather than the braided copper lead. This anneals the spring. 2) Current flowing through the brush creates a circular magnetic field around the brush itself. This stray field interacts with the motor or generator field itself. The variance in the fields caused by the switching as the brush crosses commutator segments creates a high pitched vertical vibration in the brush. Modern brush holder design addresses this by either relocating the commutator brush interface indexing relative to the fields or by using a magnetic material rather than brass as a brush guide. I believe we can eliminate #2 because not all A generators exhibit this problem.
If seating the brushes and run-in does not stop the squeal replace the brush tension springs. Check the brushes to insure the braided leads are sound. Old brushes may develop unseen corrosion where the wire is inserted, even if they are NOS. Try different brushes.
A third possibility relates to the brush composition itself. Simply put, not all brushes are the same. Some are pure graphite. The grain direction of the graphite comes into play. Brushes made from monolithic block vary in grain structure orientation. They are very long lasting IF the graphite layers are tangent to the commutator contact. Brushes made from compacted sintered graphite are much cheaper to produce but have only half the life. To increase the conductivity and reduce brush heating many brushes are made of a large % of sintered bronze or silver alloy mixed with the graphite. You see these mostly in intermittent high amperage apps, like starter motors, power tools, etc. These brushes are known to be noisy, but nobody cares for the apps they serve. If your generator brushes have any metallic appearing tint on the sides when rubbed against a piece of paper to clean the surface they are not stock spec. Again, try a different brand of brush.
