If the measured value has dropped by more than 0.5 bar:

WARNING: This page is about a different car, the 2002 BMW M Roadster, 2002 BMW M Coupe, 2001 BMW M Roadster, and 2001 BMW M Coupe. However, it is still accessible from the selected car via links, so may be relevant.

Start engine and wait briefly for a stable pressure increase.
Switch off the engine and immediately pinch off the delivery line just before the pressure gauge with the special tool 13 3 010.
Note down measured value.
Read off measured value again after approx. 20 to 30 minutes with engine stopped.