Volts dropped below 13

[xRide]

So I just changed my alternator because I was getting all kinds of message (see post...5 weeks and problems...). By changing the alternator, it seems to solved the problem. Truck running great so far. But I have been monitoring the volts. The volt go from 13.7 and 13.9....only those 2 readings come up on the screen. However, whenever I HARD BRAKE where the tranny and everything is under a sudden pressure, it dips to 13.1, one time i saw it dipped to 12.8.

Is this normal?

I do have that 2 to 1 hard gear down on the tranny.