1990 Geo Prizm Battery and Brake Light coming on??

I have a 1990 Geo Prizm LSI with a 1.6 in it. About a week ago on a cold morning the first time I went to stop and hit the brakes the brake and battery light came on. It only stayed on about 30 seconds then went off and never did it again. Well today about 2 weeks later it did the same thing right when I started it up and started driving, every time I hit the brakes the brake light and battery light would come on. The lights would stay on for about 30 seconds then go off but every time I hit the brakes they would come back on. This did this for about 10 minutes then it did not do it again. I took it by Advance Auto and had them check battery and altenator and both checked out fine, I also checked brake fulid and brake lights. If anyone has any ideas or suggestions I would appreciate them. Thanks

 

Sounds like the alternator. The autozone type test is not accurate.