There are several things that use vacuum on the 98 GT:
Power brakes (power assist)
AC/Heater control switch and vacuum motors: controls the postion of various trap doors in climate control system to direct air flow to desired vents
EGR valve: used by PCM to monitor EGR system for proper operation
PCV valve: to remove burnt oil and blowby gasses from crankcase
Fuel pressure regulator: to maintain constant pressure across the fuel injectors
Evaporative emission canister: remove vapors from gas tank and prevent them from venting to the atmosphere
I don't have the complete diagram in one place or I'd post it...
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