How does the vacuum system run on a 1991 Chevy with a 350 engine?

well it depends..usually if you have a 4brl carb the vacuum is built in the carb.

the intake manifold is the main source of vacum. from the carb mounted on the manifold. if you have a 2brl and want to switch to a 4brl, you have to get a new manafold that is made for a 4brl carb..it might be worth your while to do this..it elimates all the hoses and clutter junk from a 2brl.

if you have a 2brl, it can be a little complicated. the vac on a 2brl is threw the air cleaner unit. theres vacum hoses that conect to the rear of the carb. the EGR valve a round like cilinder on left side of intake manifold. the choke mounted on the side of carb, and the thermostat housing.

...good luck