How do I reset a brake warning light switch?

What is your question?

I have an 1985 Chevy truck. The master cylinder went bad causing the fluid to leak out of the back reservior causing the brake warning light to come on. I know the switch is located on a crossmember under the radiator. How do I reset this switch. I have replaced the master cylinder and have bled the brakes, but the light is on and I don't think I am getting full fluid pressure to the back brakes. I need to know how to reset the switch.