I had a similar problem,you need to strip down the mower casing,by removing all the cross head screws.untill you can see the motor,you will find the fast acting brake module mounted on top of the motor,undo the two securing screws and free plastic disc with some detergent do not oil this unit as it acts like a brake when the power is DE-energised,also spay some light oil at the bearing end not anywhere near the top of the motor as you dont want to contaminate the comm or brushgear.
check thet the motor turns freely with the brake unit off,if stiff check the lower bearing at base of motor.
Reassemble and use.
Flood, May 2014
