My question: How come my brake warning light on the instrument panel didn't come on when the master cylinder reservoir for my rear brakes turned up dry? Note that the larger reservoir (front disc brakes) was full.
Details:

1. I realized my brakes were taking more leg muscle and the pedal was going closer to the floor the last few drives I've taken. I checked my master cylinder reservoir to find the smaller reservoir (back brakes, drum) was empty. I know how to deal with that, and was able to bleed the rear brakes (I'm still investigating why the reservoir was empty).
2. The brake warning light comes on, briefly, when I start the vehicle, then turns off as it normally should
3. If I detach the connector that leads to the warning light, and then I touch the connector to ground (with the ignition on) then the warning light comes on.
4. I was able to bleed the rear brakes. This indicates the pressure piston is not stuck in the forward position.
5. The sensor's spring-loaded pin that rides the top of the piston is not stuck. It moves freely.

So, why didn't my brake warning light turn on when the rear brake reservoir drained?​​​​​​ As I understand things, the empty reservoir should have caused the pressure piston to move forward, closing the circuit (via grounding) that causes the brake warning light to come on.

Thanks for any suggestions.
Dan J.

Generally, the larger reservoir is for the rear brakes. (which tends to be the front one at the MC)

I would bleed the fronts as well, just for giggles, and to get the old fluid out.

Truck info might help!

you got that backwards. Big reservoir (usually towards the back) is for the front (disc) brakes.

Oops. Kind of a major omission. The truck is a 1992 4wd diesel, 3/4 ton, 6 cyclinder engine.

Yep, the fluid I bled was cloudy, so will be bleeding the front brakes as well.