The fuel preesure should immediately drop from 6.5 bar to around 3 bar when the key is turned off. The rest pressure should remain for an hour or more. (Usually a lot more).
The pressure is necessary to keep the fuel from boiling as it heats up standing still in the shut off system. If you are having this problem now (as opposed to when its really hot) you will find the pressure dropping to zero probably within minutes.
Bosch Master, ASE Master, L1
33 years MB technician