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.