Do you have a voltage-ohm meter(VOM)?. If so, do the following:
- Start car - leave all accessories off
- Set VOM on 20 scale - DC
- Place neg. prong on neg. terminal of battery
Place pos. prong on pos. terminal of battery
What kind of readings are you seeing?
Did you buy an after-market set of brushes or Bosch? Alot of the after-market stuff is junk. Was it doing this before you changed brushes the first time?
With new brushes, you should be seeing 13.8v - 13.9v at idle on first start-up in the morning.