Just to refresh, I got a free 81 300SD with no keys. I just got back from vacation and got started on it today. I got power to it and tried locking the doors, then unlocking. Not able to get in the trunk. I can hear the pump start and stop, both locking and unlocking. I yanked the back seat (no trunk release) and found the yellow hose. now if I remember correctly, pressure should unlock, vacuum should lock. How much air pressure (psi) do I need to work the lock

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

bump

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

Hook it up to a bike pump and pump it untill it opens. Go slowly or you might wreck something.

However, if the vacuum system fails to unlock the trunk, the actuator at the lock may not be functioning, in which case I'd say pull the first aid kit box for access to the trunk latch with a long stick.

Can that be done ? Isn't the fuel tank in the way?

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

You can see into the trunk, but you can't realy do much work. A long stick won't help you much. You may have to sacrifice the drivers side tail light to get to the mechanism. Here's a picture of what you're looking for if it helps. Moving the mechanist toward the passenger side unlocks the trunk. You might want to visit a junk yard to look at one before you go breaking anything. It will give you a better idea of how it looks in there & what you need to do to get in.
A bicycle pump won't deliver enough volume of air to overcome a leak if you have one in the system. A compressor with a constant volume of air would be better, just don't over do it with the pressure.

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K

The pic of the SD trunk lock you posted look very much like the W123 trunk lock. See thread below "breaking into trunk without breaking anything". May work for you if the first aid kit is in the same place.

Breaking into 123 trunk w/o breaking anything

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The stick method will be difficult at best since there's carpeting covering the part you need to access.
Can you get in touch with the guy you got the car from? If so, ask him if 1 key worked everything, or if the ign had its own key. If it's 1 key. have the dealer get you a new key from the VIN. If the ign had its own key you're going to have replace the ign lock cylinder anyway since you won't be able to get a key for it. The dealer can get you an ign lock cyl & matching key based on the VIN that will then fit everyting. Should cost about $80.00. Sounds like a lot, but when you factor in your time & what you might damage or break trying to work around the keys, it's not too bad. Even if you find the car's not worth fixing in the end, you've got way more than $80.00 worth of parts there. You can part it out to get your money back, or keep it for parts & find another.

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K