Some sparking!
The brushes do wear and I have changed mine now twice in the 19 years to 2003.
The minimum length is about 8mm, new= 18mm.
Pt No 602834 (W=7mm d=5.5mm)
I normally try to 'dress' the commutator. Last time I checked the dia. was 0.953 inches but I am not sure how much more meat is left.
I am about to investigate exactly the same problem.
Stuart T
October 2008