I found the problem. I took the 5-way connector out and boiled it in water for a couple of minutes. After re-installation, it shuts down in about a quarter second. Or, that's when it starts its shutdown, anyway. But it's immediate.

I looked into my records and saw that I had bought a new 5-way a few years ago. I also seem to remember spraying it liberally with 303. That seems to have made it less supple and less able to seal on the tube. Sure, it will never crack, but it doesn't work.

I'm still wondering why pushing the button prolonged the run-on, though.

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1982 300CD Petrol/Black Leather