Wow. I'm amazed how complicated the function of this little switch is. It is literally one of the simplest systems on the car, but I had my mind blown looking into the switch and realizing both wires were hooked together inside the switch.
So, there is another case I am wondering about. WHAT IF, there is not a good ground available on the prop valve? then, even if the shuttle valve displaces, the light won't illuminate, right? Maybe not realistic, because the prop valve is connected to the MC with hard steel lines, and so ground should pass through from MC - even if not available at the prop valve bolting to firewall.
But if there is not a good ground, the light won't work, right?
I agree Jbobo, when Randy said MC, he meant prop valve above. And, it is also amazing that the diagram you provided SHOWS the plunger in contact with the shuttle valve when centered even though it can't be touching in that condition - otherwise "light". AND the wording on the diagram implies it is a usual switch related to the plunging, when in fact is significantly different from a regular switch....
Bottom line is this: If my switch is working, and my dash bulb & wiring to the bulb are good - to test the switch, I need to provide a good ground to the "plunger" or "copper nub" on the switch. When I do that, I should get a brake warning light illumination on the dash if the key is in the RUN position.
All of this also means that the "plunger" on the switch is "hovering" in open space when the shuttle valve is centered. The only reason it is on a spring is so that it can slide back and let the shuttle valve pass by - whenever there is an imbalance in the front/rear brake line pressures and the valve moves sideways.
So, I'm gonna go see if I can find any voltage in my line when the key is in run. Right now, no matter what I do, my light won't work. But I know the dash bulb is ok (or at least it was only a week or two back).
I may be looking at a bad ground, or something not properly energizing the bulb circuit with 12ish volts.
I don't know, but I have found this conversation to extremely helpful... even if I don't fully understand the comments above related to the CRANK key position....
THANK YOU guys.