i have the 88 service manual (thanks , again, DougH !) ... it has 82 pages on the fuel system ... i'd be glad to photocopy page 14-43 (Vacuum Hose Diagram - Federal Except High-Altitude Vehicles) and mail it to you if you'd like (sorry, i don't have a scanner)

i compared your description with my manual and my rig ... you are correct about the nipples you list as 1,2,5

you are also correct that nipple 4 is capped

nipple 3 has a hose that winds down and connects to the top nipple of the six-sided thing-a-ma-jig (i'm still searching for the correct name of this doo-hicky) with five hoses connected to it ... this six-sided thingy with the five hoses connected to it is exactly 5 1/2 inches straight down below nipple 5

i hope this helps