[Ironangel]

Stomp the brakes with the bleeders closed. The OP's prop valve didn't have the indicator pin that usually protrudes when a rear wheel cylinder fails. If yours is protruding, push it back in and hold it with the tool. Then bleed the system starting with the RR, LR, FR, FL, and you should get a firm pedal with no light once the tool is removed. If you have a firm pedal and the light stays on, then give it a stomp to reset the prop valve. It's not unusual for a 45 year old system to get rust and dirt in those valves causing them to hang up. They are not rebuild-able but like the OP did, you can open the electrical switch and possibly relieve enough pressure that it will slide back to center thus opening the light circuit allowing the light to go out.