I had the same light dimming issue when I hit my brakes. I replaced the alternator and I think it help a little, but the lights still dimmed it was driving me nuts. I replaced my battery with the Optma Yellow top. And I also put a lightning cap on the power to my sub's amp. I have not seen my lights dim in a long time.
Hope this helps!

 

Hey 92 Lex - what'd you upgrade to? Which manufacturer, which model, etc. What amperage, most importantly?

Thanks -

Matt