89 300e key won't turn in exterior locks

[Lyman4]

I bought an 89 300e in january, and the key doesn't operate any of the exterior locks (doors or trunk). the locks, as well as the central locking system works if you push the locks down manually from the inside of the car, but the key simply won't unlock or lock them from the outside. The key works fine in the ignition and glovebox. My first guess was that a previous owner lost the key, and swapped the ignition and was using a key from the donor car, seeing as though none of the exterior locks worked, so i ordered a new key from the dealership, and it matched the key i already had, and still doesn't operate the door locks. with the passenger door open, i press the lock lever down, and all 4 doors lock with vacuum pressure, and with the key, i can turn it about 20 or 30 degrees, and the lock lever raises up, but not quite far enough to unlock it. in the drivers door i get a little less action, but it can turn about 5-10 degrees. it seems unlikely that all three exterior locks would have been replaced, given that the original key was still in hand, and the previous owner told me that it just stopped working one day. could there be some kind of vacuum back-pressure keeping the cylinders from turning? I've tried both wet and dry lubricant. i suppose my next move will be to try removing the locks themselves for further inspection. any advice?