I have a 1978 300d and was having problems with the door locks. So I recently had the vacuum hoses changed. The doors now lock and unlock well with the car running and once I turn it off, I can only lock and unlock the 4 doors once. I not only changed the hoses, I also changed both passenger and the rear drivers elements and the vacuum lock cylinder, as well as the trunk and fuel door elements. The vacuum chamber was also changed. I haven’t changed the gray vacuum hose or anything on the reserve tank. But the problem now is that I can’t get the fuel refill door open (with or without the car running). This in time will lead to another problem, no diesel. I am not sure what could be wrong. The vacuum pressure was checked and all checked out fine. Any help would be greatly appreciated.[/align]

 

I'm not sure, but can't you access the fuel door lock from the trunk? It sounds as though its sticking from coldor bound up from when you replaced the elements. I hope this helps some!

 

I'm not sure, but it sounds like you are loosing vacumme when the car is off. Try putting a guage in-line and check that the vac holds. You might see of the lines to the reserve tank (in the trunk) were properly connected after the lines were replaced.