I'm not sure if your locking system is like mine or not, I have a 82 300D turbo, but if you check out this thread

82 300TD A/C problem
you will see how I chased down my leak. I found on mine that the component that leaked sometimes leaked off enough vacuume while the engine was running that other components did not work correctly. The part at fault was not the one that was acting up. I hope this helps you out.
Rick