what about a simple rear brake adjustment? if you pump up the brakes the shoes contact the drums then you have lots of pressure, one pump and there's not much there.
do the brakes seem to get better as you pump? or do they seem to be the same from the first pump to say the fifth?
if the get better it's probably air or an adjustment in the back, if they stay the same i would look for a vacuum leak.
i had a leak and it your right after further thought it didn't run rough but the idle speed did increase by about 200 - 300 rpm. ran around 900 rpm. mine was a direct connection to the manifold (pretty sure all are) because boost is needed from when the truck is started and the amount needed is not variable (full amount is needed at all times).