Does exhaust feedback cause carbon brush to wear abnormally fast?

[Tea]

I have an AC series motor (universal motor) with a fan mounted on the output shaft, used as a vacuum motor. This motor is put into an enclosed container (bucket). There is a seal inside the bucket to seperate the intake air from the exhaust. In one incident leakage (20 liters/minute) happens at the partition where the exhaust is able to feedback to the inlet. When I take out the motor and turn it on I observed heavy sparking. Also, I found very fine orange color powder at the bottom of the bucket and noticed that the brush has worn abnormally fast. I believe that the fine orange color powder is the coating on the carbon brush. This motor has an thermal cut off (TCO) protection and it does not trip when it was tested. This means that the motor was not overheated. Also, the motor TCO temperature is not too much different (5 deg C) compares to peer motors under the same test which do not experience exhaust feedback. Can anyone tell me is it the exhaust feedback causing rapid wear of the carbon brush? Why is it so?