74 duster brake light switch

Pardon the interruption, but let me throw in my $.02. The block you are describing has a piston in a chamber between the front and rear circuit. As long as there are no leaks the piston stays in the middle of the chamber and the brake light stays off. When a leak develops and the piston gets to the end of its travel, the light comes on. Just below the terminal for the wire in question, there is a brass plug. If I remember right it's 1/2" hex. Remove it and you can reach in with a pocket screwdriver and work the piston back toward the center of its travel. If that don't work the large flat hex plug on the end of the block will give access to take the whole thing apart. Reset the piston and if you haven't found a line or hose leak before the light comes back on, it's the master cylinder. When bleeding DO NOT push the brake pedal to the floor. If you go farther than halfway to the floor the silly thing trips and you have to reset it. Hope this helps.