I have a similiar issue on Coda, my daily driver. If I know I am going to be opening the trunk when I turn off the engine; I will lock the doors from the inside while still driving by pushing down the driver door lock "button". Then I will unlock it, before stopping. It hasn't failed me yet.

In addition, if parked in the driveway with the engine off, I just get in the car, push the driver door button down. Then pull it up. This has not failed me yet, either.

Obviously I have an issue with the trunk lock mechanism, but no vacuum leak in the system elsewhere.


Does your fuel door lock and unlock with the doors?
Another idea for your 300D, might be to find the vacuum hose that goes to the trunk (and fuel door) under the passenger door mat near the seat, outside corner. If you sit in the passenger front seat and have your knees at 90 degree angle, the connections should be to the outside of your right heel. Hook up a Mity Vac to the hose and give it a LOT of vacuum.

__________________
daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
No longer selling Cluster Needles Paint
No longer selling New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more