M45x AWD V8-4.5L (VK45DE) (2008)
Use a handy vacuum pump to check.
When connected to the booster side (1):
Vacuum should decrease within 1.3 kPa (10 mmHg, 0.39 inHg) for 15 seconds under a vacuum of -66.7 kPa (-500 mmHg, -19.69 inHg).
When connected to the engine side (2):
Vacuum should not exist.
^
Replace vacuum hose assembly if vacuum hose and check valve are malfunctioning.