said by EGeezer:
It's also reasonably possible (but not probable IMO) that a brake light bulb may be shorting out.
Then the fuse would blow.
The voltage drops when the brakes are hit and the car is stopped because:
1. The alternator produces less current at idle speed
2. The brake lights are a fairly high drain (2x25W give or take)
3. The battery is weak
It isn't impossible for the alternator to have a problem (blown diode), but that is difficult to test without proper equipment and without removing the module. It is far more likely for a 5yr old battery to be weak.