If the measured value dropped by more than 0.5 bar

WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2005 BMW 325i, 2005 BMW 325Ci, 2004 BMW 325i, 2004 BMW 325Ci, and 2003 BMW 325i. 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 wit special tool 13 3 010
Write down the measured value
Read the measured value again after about 20 to 30 minutes at engine standstill