Need Help - Trunk contents held hostage ('79 280CE)

Hi all, if anyone has any advice, then I'd greatly appreciate it. The trunk of my '79 280CE will not open. The key will go into the key slot, but will not turn. I can depress the trunk-release button, but
that does not allow the trunk to open.
There are items in the trunk that I need to get to, so even if I can't get the trunk open...then is there another way to get into the trunk, maybe through the backseat area?
I've gone to a local Mercedes-Benz mechanic, but he has way too many cars to work on before getting to mine...and he said I need to make another key at a locksmith and try that first. The key I already have works fine for the doors, just not for the trunk.
Any advice would be greatly appreciated.
Thanks all!!!
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WD-40? Is that car also run by vacuum tubes? If so, put an air compressor up to one of the vacuum tubes, and see if they will unlock it. I did that once from the trunk to open my doors when I locked my keys in.

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MBs come with two keys: a Valet key and a Master key. The Master key will lock and unlock the entire car (including glove box and trunk). The Valet key will only open the driver's door and ignition lock; it cannot open the glove box or trunk.
It sounds like your trunk was locked with the Master key, and now you're using the Valet key to try to unlock it. You need the Master key for the trunk and glove box if you want to manually unlock them. The central locking system won't unlock a manually locked trunk, only the Master key will. It's for securing valuables when leaving your car with a parking attendant -- you hand them the Valet key so they can enter and start the car, but are unable to access the manually locked trunk for anything inside.
The key sets used to be marked with red circles or some symbol to differentiate between Master and Valet; try another Master key and see if it doesn't open the trunk.

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In decided to enlighten our sheltered souls with a rant as follows

easiest way to tell, if the key your using has a groove down the middle of the actual metal bit that goes in the lock, it's a master key, if not, then it's the valet key.
Another way to tell, if it's the original Mercedes key (merc star on shaft) if the head is round in shape, it's a valet key, if it's square, tis the master.
--
Pete M

Alfa 164 TS,
  Click to see the full signature.
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Hi everyone, just wanted to say thanks for all of your tips. I especially like the air compressor idea, and I'll keep that in mind for the future.
The key I have is the master key, so that wasn't the problem. I went to a local MB dealer and they tried WD-40, but that didn't work and they also referred me to a locksmith...so I figured that was the only way. The locksmith made another key and then just turned it really, really hard (using a wrench to turn the key with more force) and it worked. Unfortunately, the stuff I thought was in the truck wasn't there...but that's another story.
The lock still doesn't work correctly, but at least I have an extra key and keep a wrench in the car for the next time I need to open the trunk. I'll just add that to my list of "things that need to be fixed". Fortunately, that MB is not my primary car...so not having trunk access isn't so crucial. There are plenty of other things that need to be fixed first.
Thanks again for all your help!!!
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