The brake switch closes (goes on) when there is a significant pressure difference between the two sides of the duel hydraulic system for the front calipers. If that happens the switch closes and is supposed to turn on the brake warning light on the dash. On a '76 two other things turn on the light as well: the low brake fluid switch or the parking brake being set. One thing to determine before resetting the switch is what caused the switch to be turned on, usually it goes on when one is bleeding the brake fluid, or there is some sort of leak in the braking system. To reset the switch find it below the brake fluid reservoir. You will notice it is has several brake lines going to and from it, and has a pair if wires attached. Right between the wires is the switch. Originally there was a small rubber boot over the switch. Under the boot a small square piece of plastic extend when the switch is on. To reset the switch push the plastic back into the switch. If all else is working the brake warning light should not be on. Sometimes the plunger inside the switch gets stuck and needs to be freed. To do this one can disassemble the switch, clean it and try again. An interesting device the brake warning switch! Have fun and let us know how the solution to your problem.