With the engine off, how long before the stop lever goes back to normal position? When it goes down by turning the key to stop the engine, the lever should go down and stay down until you turn the key to glow. Days... weeks, even.

With the engine running, disconnect the hose from the brake booster line to the key switch and hook it up directly to the shutoff actuator. If the engine stops, there's a leak between the vacuum source and the actuator. If the engine doesn't stop and you can't be convinced to obtain a MityVac, try a new shutoff actuator and work back from there.

Sixto
83 300SD