Charging System Troubles

I have an '87 Monte Carlo SS with a 327 and Th-350 Trans, my problem is that my alternator decided to die one day while I was driving. I saw an unusual drop in voltage from 13 volts to 9 so i parked the car and left it for a couple days, I tried to start it and nothing (this all happened right after my starter died, I replaced that and it was fine, then about two weeks later the alternator started giving me problems) so i charged the battery and took the car to school i tested it and i was getting 31v of AC current back to my battery, so either my diodes or rectifier bridge was toast. i replaced it with another alt from a Monte i had in my garage, same thing happened, AC current coming back. I bought a Valucraft from Autozone, same thing 11.5v AC back to the batt, returned it and got a duralast and i haven't tested it yet but it's definitely not charging the battery. right after i got it I drove the car for about 4-5hrs at an avg speed of 40mph and nthn my battery just gradually lost power, not nearly as bad as before but its still screwed up, so my question, is there anything else in the charging system that could be causing this if there is I had no idea about it whatsoever.