Arcing Brushes

I just purchased a used MM1800 Type 1 corded mower for my daughter to use on her small patch. While cleaning it up and checking it out I noticed that when the power lever is released there is a bright arc from each brush. The arcs are bright enough to be seen through the orange plastic cover. Otherwise the mower runs fine. The brushes look OK with about 1/2" of each brush remaining in the holder. The commutator also looks OK. I've never seen this from and electric motor before and I'm skeptical that it is normal. Having said that, I took the cover off of the MM875 that I use to see what it does and it does the same thing, but the arcing is not quite as bright. I've never noticed it before because the cover is black. I'm thinking of contacting the local service center about it next week, but thought I would check with you folks first.
Thanks