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You may have a disconnected or broken vacuum line.
You can find this schematic (and others) in the Chilton's manual for your truck.
where dose the vacuum under the battery come from
look on e bay, they had a use Nissan service manual for 5.00
AUTOZONE.COM
If you are talking about the emission control system vacuum schematic, purchase a Haynes manual and turn to page 6-2.
The vacuum diagrams are too large for me to post here. But you can get them at the Public Library in the Reference Section.
Mazdatruckin.com has a schematic for the B2600i vacuum lines in their tech section, it includes 4wd and cruise control devices. See the attached links below.
The AC system from a newer Nissan 300zx can be retrofitted to a 1987 with a serious vacuum problems by making a number of modifications. New mounting brackets, wiring, refrigerant, and AC hoses will be needed.
You can find a detailed vacuum hose schematic for a 1984 Oldsmobile Cutlass Supreme in the car's owner's manual. You can also check with local auto stores.
Inspect all the vacuum lines, you may have one cracked and you are loosing vacuum to operate the HVAC system.
Need more information.