The 928 has some actuators that require vacuum to operate. But after 25 years or more the rubber goes brittle and my leak or tear.  Some actuators for the 928S4/GT/GTS:

- center vent opening, footwell vent, defroster vent, recirculation vent

- tank breather valve, diverter valve for the aux-air pump

- 2x fuel pressure damper, 1x fuel pressure regulator

- "Flappy" valve inside the intake

- pressure modulator inside the automatic transmission

- early 928 cruise control actuator

So there is an obvious need to measure and check.

A perfect tool for diagnosing is a Mityvac. I bought one and am very happy with it. It is like Porsche tool P9160.