W115, Repair Kit

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?

Just replaced the oil filter DIY first time. I didn't see where one of the small pressure washers should be installed. Also, how do you replace the pressure washer between the bolt and the filter housing? Thanks.