It's not an emergency brake wire, it goes to the combining valve by the master cylinder and when the fluid in either side of the master cylinder is low, a switch in the combining valve grounds the wire and turns on the light in the dash. If you ground the wire to the master cylinder with the key on the light should come on and it You turn the key switch to the start position the light should also come on.
Here's a picture of the wire on the switch.
