The 601/2/3 stop lever should move with the vacuum key switch. Turn off the engine and it should move down and stay down until you turn the key to glow. How long it stays down reflects the integrity of the vacuum system. In my blue '87 300D it would stay down for at least a couple of weeks. IIRC in my white '87 300D it wouldn't stay down more than 24-48 hours. If the engine shuts off with the key but the lever doesn't move down, it's not properly attached to/in the IP.

Sixto
83 300SD