I didn't think that was a problem with the brushes, but a sticky solendoid.. have you tried taking it apart and cleaning it up, that way you will at least be able to remove the brushes and check them, they were £6 for an alternator a few years back.
I replaced the brushes in my van a while back. It was a simple enough job. I cleaned all the dust out from inside the motor housing, cleaned up the commutator (motor contacts that the brushes touch), greased the bearings and it worked fine since then. Thats what I'd do if I were you. Its not that compicated.
Yeah i replaced them. Its mad not to if you ask me, unless theyre hardly worn at all. Theres an electric motor rebuilding place in Dublin that have all sorts of different sized brushes. Thats where I got mine from.