Alternator question

What is the power consumption of your stereo?
What "other" electronics?
What kind of brakelight bulbs?
The point-type ignition system only draws maybe a couple of amps, and the brakelites even less, leaving plenty left over for a typical stereo. But that's only half of it; if the alternator died,the battery should power a typical stereo for hours.
I would check the amp-draw of the brakelites, there may be a short in that circuit.
in the meantime;
Crank the idle speed up, and don't step on the brake :)