Looks like the MC has a plug on the Clutch side....

As for you brakes, look under the cab for a boooster, there should be a Vac Booster under the drivers side, the MC on the filre wall just operates the under cab booster.

You need to spend some time looking over this website for more info about these bigger trurks. http://www.chevyc60.com/ there is even a page for rebuilding the brake booster...