Below is the vacuum diagram for a truck with an XSS EPA code. I knew the engine size and year, but without any more info I had to guess on some stuff. In addition to 305/1984, I used automatic transmission, federal & low altitude. "Federal low altitude" just means not California and not high altitude (essentially the standard emissions package). But it really doesn't matter as far as this particular issue is concerned - the vacuum diagram for every 1984 with a 305 and AT had only one hose connected to the back of the carburetor. And in every case that hose went to the EGR bleed solenoid. The other hose is probably the vacuum supply to the power brake booster.
