brakes

I hope you use DOT 5 or else clean the spills up quick. That is real purty paint job to ruin.

The brake warning light probably comes from the "pressure imbalance switch", which is built into the proportioning valve that you should have for a disk-drum setup. I don't think your MC or car has a "low fluid" warning switch. Search here for a trick on how to center the imbalance switch. I have never done so, but recall you need to crack a nut or such as you push the pedal and bleed some off.

I have no idea why the reservoir would overflow when you push the brake pedal. It sounds like maybe a problem in the MC. I haven't studied them, but I expect they are basically a piston with a check valve. The check valve allows the piston to draw fluid from the reservoir as you relax the pedal, then blocks it flowing back to the reservoir. Sounds like that isn't working, so you get a spurt in the reservoir when you press the pedal, instead of pressure.