I have been going through all the locks on my 77 Transporter and getting all locks to work with one key. Have completed restoring the locks for the hatch and sliding door. The key tumblers were filthy and took considerable time to clean and lube. I used some teflon based spray to lube the key cylinders.
I am now doing the passenger door, and the tumbler was the same story, but all good now. On to the lock/latch mechanism.
You can see from the photos below that inside the door was very filthy and the door lock mechanism itself was just caked in old grease and dirt. I have cleaned and cleaned, and it now operates smoothly. Door cleaned up as well.