No, I'm not talking about the charcoal cannister, that's emissions. This is some sort of reservoir(they call it a reservoir, but it functions more as an accumulator)in the vacuum lines to provide a steady source of vac. when the engine vac drops. Usually located on a fender apron or in some out of the way corner of the engine compartment. I've worked in a GM shop too long to be able to say exactly what it looks like or where it is, but Ford used to be good about using what looked like a
1 lb coffee can painted black with a hose going to one end of it. GM uses plastic ball-looking things.-- Old Crow '82 FLTC(P) 92" '87 FLTC '61 F-100 302/C-6 BS#133, SENS, TOMKAT, SLOB#13, MAMBM