'Nuther thing the pressure control solenoid does: it allows for wear to the clutch assemblies. The engine/powertrain computer 'learns' what is needed to provide solid shifts. As the clutch packs wear, the PCS ups the line pressure to maintain solid clutch contact and prevent slipping. This constant adjustment eliminates clutch slippage due to wear and provides great shifting throughout the life of the clutch pack. Basically, the tranny shifts great til it doesn't. Replacing the PCS with a vacuum modulator eliminates the 'learning' ability built in to the transmission. You're pretty much taking a step backwards.
If you're installing a 4L80E in a non-computer vehicle, consider using the stand alone powertrain control module used in the P-series 6.5 vans thru the 90's. Mechanical injection pump, computer-controlled tranny.
