Converting from single master cylinder to duel ?????

I put a dual master cylinder (MC) in my 65 Dart, but mainly because I was redoing the brakes and wanted to allow for later front discs.

Re safety, I understand how a dual MC mitigates the risk of losing all brakes from a failed line. However, a dual MC doesn't prevent losing all brakes with no warning, in my experience. My 69 Dart had a dual cylinder (drum brakes), yet at a stoplight the pedal went to the floor. I had to pump the pedal to get brakes. I saw no warning light before. I recall the light on after, but maybe as I pulled the E-brake. It had a switch in the distribution block, which is supposed to turn on the light if front and rear pressures vary (assume), so either the switch/wiring was bad or both front and rear MC piston seals failed simultaneously (unlikely). Perhaps the former and I had been driving with no front or rear brakes for a long time until the 2nd seal failed.

My '65 Dart uses an MC from a 99 Breeze, however I don't have a similar "pressure imbalance" warning switch. Perhaps newer cars include that in the ABS block. I probably should have plumbed my setup with a distribution block from a later Dart, though I would need to mount it high since the rear proportioning valve follows and I don't want that down in the dirt. Another choice is to switch to the more expensive after-market proportioner that includes the switch. I'll put on my list. I am still better than my 65 Newport daily driver with its single MC. BTW, it also went to the floor at a stoplight, with a fairly new rebuilt MC. Took it apart and found a sharp port edge had cut the seal, perhaps from re-boring without filing the edge (can't trust rebuilders).

A question for experts. My new MC has a sensor under the reservoir to warn of low brake fluid (I assume). How does it works? It must be a magnetic signal since it slides in under the plastic reservoir without fluid contact. Inside the reservoir is a plastic part between the front and rear reservoir that can rock a bit. I assume it has a magnet or piece of steel. Is just a simple on/off mechanical switch or does it require high-end electronic condition to sense (AC excitation or such)?