I never had this problem before the alternator go south the voltage was steady I changed the alt. 3 times and I noticed this problem when I turn on the headlights the voltage drops little but as soon as I hit the brakes it drops more the car has March pulleys but I didnt have this problem before it was fine the car is '92 Z28 Im hopeless to be honest and Im thinking of removing the pulleys and put the stock back on some more info about the car it was fuel injected conveted to carb with no problems at all even with the high compression 383ci and the pulleys the voltage is steady put stock 350 back on and when the alt. went south and changed the alt. 3 times all of them are working and another thing I noticed that the volt gauge reads 3/4 of the gauge I might take a video if you guys didnt get what I mean

 

You did not have the problem before what? Before the present time or before the march pullies? Personally I don't think those pullies are worth it!

There is a voltage sensing wire on your starter. It will be one of the fuesable links. Don't just look at the link and think since it looks good its fine. Give a little tug as they will corrode inside.
Also have your battery tested. I just recently had a bad one. My volts would drop (the five 10-watt third brake light bulbs did not help) when I stepped on the brake. I also knew there was something wrong when I would see my volt gauge flicker. I used my battery tester (the old smoke box) and it tested bad. I drove to my brother's work where he has a nice electronic tester and the battery tested bad. Would measure 12.3 volts which is a half charge. Replaced the battery, 12.54 volts and no more flicker of the gauge.

 

i would also recommend getting the battery charged and tested. if you had a bad alt before and the battery was never charged back up, the alt's doing double the work to try and run you car and charge your battery.

are your pulleys an under drive setup or just a visual thing? underdrive pulleys will cause the alternator to not spin as fast as stock (unless there was an overdrive pulley included for the alt as well), which can cause less charging to occur at lower rpms

 

before the alt went south the car had the pulleys before the alt went south and will check the battery soon the one it has right now is AC delco the copy one that were not made in USA

 

the pulleys are under drive set up and the alt has smaller than stock pulley it was working fine before the alt went south no voltage drop no nothing and I will check the battery later when I brake and rev the same time the volt will not increase