W115

220D vacuum line tap

I have a '71 220D with a mystery vacuum tap on the main line between vacuum pump and power brake booster.
It's unconnected, and there's also a disconnected hard line up by the firewall, but it doesn't have the length to connect to the brake-line tap. (These are the only vacuum fittings on the car.) That line is split by some sort of check valve or diaphragm/filter, and it disappears under the driver's side fender, right under the windshield corner. It's green, and it doesn't seem to reappear under the dash.
I thought at first it had something to do with the windshield-washer pump, but that's foot-operated, and I've successfully reconnected that.
Any idea on what the mystery line/tap is? I've got some MercedesSource hard line and fittings to reconnect things, but how and what?

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'76 300D No start

Have the glow relay manual override installed. Last week I pressed the button, got glow for a few seconds, then a fast clicking. Was able to start the car. Now I get fast clicking & no start.

Battery tests fine. Tried powering the starter directly by jumping to the solenoid but I only get a click.

Not sure how to troubleshoot from here. Any tips would be appreciated!

Thanks,
Nicole

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