The I robot runs 1+ min. and stops with the message clean brushes. I have done this over and over, I even put in a new brush,with the same results. I have a 551 robot.
It seems you are having problems with either the brush motor, the brush deck gears or the brush deck.
We would suggest you to clean the brush deck casing gears first. Use this linkto dismantle the brush casing and reach the gears.
Clean them and put the robot back together and give it a try.