If the measured value dropped by more than 0.5 bar

WARNING: This page is about a different car, the 2013 BMW 550i GT, 2012 BMW 550i GT, 2011 BMW 550i GT, and 2010 BMW 550i GT. However, it is still accessible from the selected car via links, so may be relevant.

Start the engine and wait briefly for stable pressure build-up
Stop the engine and immediately disconnect the feed line just before the pressure gauge with special tool 13 3 010
Write down the measured value
Read the measured value again after about 20 to 30 minutes at engine standstill