[URL]https://www.web.imperialclub.info/Repair/Lit/Master/227/Page06.htm[/URL]1966
I think the thing to do is study the circuits for your specific car to figure out why it works that way on a '70 a-body.
Like 67Dart posted, on a 67 the ign 1 from the key switch is dead with key in start. On a 67 Barracuda (ralley dash) the brake warning lamp is powered from the I1 terminal. It can turn on with either the parking brake or the distribution black switch grounding. I do not recall it ever lighting for a self test upon start. It works when I pull the hand brake and the key is in run.