'69 c10 mech. question.

[philB]

this is a great site with lots of info for projects, mech. question, have a '69 c10 2wd auto with sbc, dont know what size, but there is a switch on the firewall with a vac hose from engine and small hose push on on other side, no hose if it may not need it, and wire connector that goes to the break switch under dash, dont know what this does and cant find info in Haynes manual, is this something needed or just whats is it for,