20" Hg sounds great. I think you only need ~10" Hg for most things to work (door locks, engine shut-off), though the tranny shifting is proportional and fussier. You can rig up tubing to measure only the main tube from the pump, with nothing else connected. That is the best way to check the pump alone. You can easily find info here to remove the EGR controls, as many do, if that is a leak source. Good luck, rebuilt pumps are >$300 and even the simple rebuild kit is $100. I also have an 85 and the CA model. The vacuum controls aren't that complicated once you go thru the diagrams you find here. Main addition in 85 was the vacuum amplifier "blue saucer" that is supposed to improve the transmission shifts.