ewmROVER, The part you show in your first post is a pressure operated micro switch, and not what you want.

There are a couple ways of doing what you need.
One would be to get a manual, lever operated valve, with connections suitable for the tubing size. Operating the lever (or however it is operated, could be a button, but I haven't seen a button operated valve) opens the port, allowing vacuum to go to the fuel pump shutoff valve.

The second way to do it, with the key is to get a 12VDC solenoid operated valve. These are usually NO/NC like a spdt switch. When the key is turned on, the solenoid closes the ports you want to be connected to.
When the key is turned off, the solenoid is de-energized, and the port then opens, allowing engine vacuum to go to the pump shut off.
Connect the vacuum supply to the NO (normally open) port, and the line to the pump to the common port. This will ensure that you're not sucking air when the key is on.

What Landie are you puting the engine in? We're also into them, and are now down to 7 of them. Did you get one of Jim Young's adapters? I'm toying with a 300 turbo into my 109. I'm getting ready to put a 200Tdi into the 88. I love the 4.2 I built too much to change engines in the 110(#103).
Terry
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