Do you have a Mityvac?

The engine shutoff system consists of source vacuum, a selection valve at the key switch and the vacuum actuator that does the job of pressing the STOP lever. A simple test is to apply vacuum directly to the actuator to see if the engine stops. If it does, go to the other end. Apply vacuum to the brown line at the brake booster vacuum supply line check valve and see if that stops the engine, Be careful handling the check valve. The nipples will be as brittle as the vacuum lines in the engine bay. AFAIK you have to buy an entire brake booster line; they don't sell just the check valve.

My 617 days are long ago. I don't remember if the STOP lever moves with the vacuum actuator as it does in the 603. If it does, no need to have the engine running. Just observe the STOP lever moving to confirm what you expect.

Whatever you spray in the vacuum system in these cars ends up in the engine crankcase. I wouldn't want WD-40 in there and certainly not carb cleaner!

Sixto
87 300D