I have the same red light problem on one of two identical brushes. If too much pressure is applied the contact is open the red light comes on and less power is applied to the motor of the brush.
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The bad brush is more flexible when even minimal pressure is applied the red light comes on. The deffect comes from too much flexibility.
I took the motor assembly off on the wrong blush and found out that some solder points broke there are four solder points on each side of the motor circled in red.
Solution is either to glue or solder if you have the right equipment. I personaly used a rubber band around the brush close to the red light to tighten the head.
Hoping that helps.