I came across a brake junction block while sorting through some old parts. I took it apart and photographed the inner workings.

Of course the wire coming to the switch is a 12 volt lead coming from the brake failure light. In a system that is working properly the pressure is equal on both sides of the slide. In a system with equal pressure the contact pin rests on the part of the slide that is non-conductive which deprives the circuit of a ground so the light remains out. If the equal pressure on both sides of the slide is disrupted the slide moves towards the low pressure side. This would occur when one of the two circuits in the master cylinder failed or pressure is lost in one circuit for some reason. When the slide moves enough to allow the contact pin to complete the circuit on the conductive material (blue arrows) on the slide the brake failure light comes on.

The brake failure light will remain on until the slide is moved back enough that the contact pin once again rests on the non-conductive portion of the slide.

Unscrewing the plug and removing the contact pin (and spring) is easy enough. The plastic plug has a slot formed in its top making it appear that it just screws out. That is not the case. It is necessary to find a tool that will fit through the hole in the plug and has a 90 degree bend at the end. If you insert the tool and then pull up you can remove the plastic plug. I had a dental tool in my box that worked well for this. With the plug out you are now down to the slide. Using a dental pick I found it easy enough to move the slide back to center as long as I was able to get a "purchase" on the non-conductive portion of the slide. The dental pick was of no use when the slide had moved far enough to make the rubber based center inaccessible. Of course I had the luxury of having the block off of the car and on the bench. Imagine how difficult this would be with it still in place under the car.

Another method of successfully centering the slide was to remove the 21mm plugs at each end of the junction block. Simply remove the plugs and determine which way the slide had moved so you could push on the other side to move it back. Then make sure you move it back just enough to center it so the contact pin is resting on the non-conductive center of the slide. Also bear in mind that with the plugs out the brake fluid runs out and it will be necessary to bleed the system once you get the plugs back in place. There is also the chance that you might damage one of the Orings that is supposed to seal the fluid out of the area that contains the plastic plug, contact pin and the wire contact.

Which ever method you might choose to try to "turn off" the brake failure light will not be easy, but it is doable.