The tailgate in my 1984 300TD locks by vacuum, but doesn't unlock. I have to unlock it with the key.
It's not a vaccum leak, I can leave the car sitting for a few days locked or unlocked and there's still enough vacuum to do 4 or 5 lock/unlock cycles. When I unlock the doors, the trunk make a loud CLICK noise. With the inside panel of the tailgate removed I can see the vacuum piston moving if someone turns the key in the driver door, but it doesn't pop the lock.

The whole lock assembly is ridiculously tightly packed into the tailgate and I've been afraid to try and remove it, especially if there's an easy way to fix the problem WITHOUT removing that mess.

I should also note, turning the key in the tailgate is a bit stuff, takes more force than it probably should, I lubed the cylinder with graphite lube and sprayed the internal latch mechanism with silicone lube, it helped very slightly.

__________________
1984 300TD -- summer daily driver
Many others that aren't Mercedes...